Jupyter notebookで”Connection failed”が出てカーネルが起動しない

前提

Jupyter notebook上でカーネルが起動しなくなりました。
以前はできていたのですが、数ヶ月ほど経って起動したところ以下のエラーにはまってしまい途方に暮れています。

実現したいこと

-Jupyter notebook上でカーネルを起動したい。

発生している問題・エラーメッセージ

ターミナルで$jupyter notebookと入力し、起動後、.ipynbファイルを開くことはできるのですが、以下のエラーメッセージがポップアップし、カーネルが起動できません。

Connection failed
A connection to the notebook server could not be established. The notebook will continue trying to reconnect. Check your network connection or notebook server configuration.

ターミナル上では以下のエラーメッセージが出ています。

~~~~~~~~~中略~~~~~~~~~~~
ERROR:tornado.general:Uncaught exception in ZMQStream callback
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 431, in _run_callback
    callback(*args, **kwargs)
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 283, in dispatcher
    return self.dispatch_shell(stream, msg)
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 233, in dispatch_shell
    self.pre_handler_hook()
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 248, in pre_handler_hook
    self.saved_sigint_handler = signal(SIGINT, default_int_handler)
  File "/usr/lib/python3.6/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread
ERROR:tornado.general:Uncaught exception in zmqstream callback
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 452, in _handle_events
    self._handle_recv()
  File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 481, in _handle_recv
    self._run_callback(callback, msg)
  File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 431, in _run_callback
    callback(*args, **kwargs)
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 283, in dispatcher
    return self.dispatch_shell(stream, msg)
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 233, in dispatch_shell
    self.pre_handler_hook()
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 248, in pre_handler_hook
    self.saved_sigint_handler = signal(SIGINT, default_int_handler)
  File "/usr/lib/python3.6/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread
Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel_launcher.py", line 16, in <module>
    app.launch_new_instance()
  File "/usr/local/lib/python3.6/dist-packages/traitlets/config/application.py", line 837, in launch_instance
    app.start()
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelapp.py", line 477, in start
    ioloop.IOLoop.instance().start()
  File "/usr/local/lib/python3.6/dist-packages/tornado/platform/asyncio.py", line 199, in start
    self.asyncio_loop.run_forever()
  File "/usr/lib/python3.6/asyncio/base_events.py", line 425, in run_forever
    raise RuntimeError('This event loop is already running')
ERROR:tornado.application:Exception in callback functools.partial(<function ZMQStream._update_handler.<locals>.<lambda> at 0x7f84220ae8>)
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/tornado/ioloop.py", line 741, in _run_callback
    ret = callback()
  File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 536, in <lambda>
    self.io_loop.add_callback(lambda: self._handle_events(self.socket, 0))
  File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 452, in _handle_events
    self._handle_recv()
  File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 481, in _handle_recv
    self._run_callback(callback, msg)
  File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 431, in _run_callback
    callback(*args, **kwargs)
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 283, in dispatcher
    return self.dispatch_shell(stream, msg)
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 233, in dispatch_shell
    self.pre_handler_hook()
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelbase.py", line 248, in pre_handler_hook
    self.saved_sigint_handler = signal(SIGINT, default_int_handler)
  File "/usr/lib/python3.6/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread
RuntimeError: This event loop is already running
[I 22:05:12.386 NotebookApp] KernelRestarter: restarting kernel (1/5), new random ports
Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel_launcher.py", line 16, in <module>
    app.launch_new_instance()
  File "/usr/local/lib/python3.6/dist-packages/traitlets/config/application.py", line 837, in launch_instance
    app.start()
  File "/home/jetson/.local/lib/python3.6/site-packages/ipykernel/kernelapp.py", line 477, in start
    ioloop.IOLoop.instance().start()
  File "/usr/local/lib/python3.6/dist-packages/tornado/platform/asyncio.py", line 199, in start
    self.asyncio_loop.run_forever()
  File "/usr/lib/python3.6/asyncio/base_events.py", line 425, in run_forever
    raise RuntimeError('This event loop is already running')
~~~~~~~~~中略~~~~~~~~~~~


[W 22:05:30.514 NotebookApp] KernelRestarter: restart failed
[W 22:05:30.514 NotebookApp] Kernel d04f7470-51ec-4402-bda1-665ca583b095 died, removing from map.
[W 22:06:09.683 NotebookApp] Timeout waiting for kernel_info reply from d04f7470-51ec-4402-bda1-665ca583b095
[E 22:06:09.688 NotebookApp] Error opening stream: HTTP 404: Not Found (Kernel does not exist: d04f7470-51ec-4402-bda1-665ca583b095)
[W 22:06:11.134 NotebookApp] 404 GET /api/kernels/d04f7470-51ec-4402-bda1-665ca583b095/channels?session_id=d637421b42d34d6788cf86d17a855140 (127.0.0.1): Kernel does not exist: d04f7470-51ec-4402-bda1-665ca583b095
[W 22:06:11.171 NotebookApp] 404 GET /api/kernels/d04f7470-51ec-4402-bda1-665ca583b095/channels?session_id=d637421b42d34d6788cf86d17a855140 (127.0.0.1) 43.240000ms referer=None
[W 22:06:14.128 NotebookApp] Replacing stale connection: d04f7470-51ec-4402-bda1-665ca583b095:d637421b42d34d6788cf86d17a855140