I have just installed Jupyter over pip (Python version is 3.7.2) in Windows 10, started the jupyter server by calling jupyter notebook, created a new notebook with kernel python3 in my web browser but a connection to the kernel is never achieved.

No obvious error in the command line:

C:\Users\xxxx>jupyter notebook
[I 21:18:21.005 NotebookApp] Serving notebooks from local directory: C:\Users\xxxx
[I 21:18:21.005 NotebookApp] The Jupyter Notebook is running at:
[I 21:18:21.006 NotebookApp] http://localhost:8888/?token=5743fcbbc805efa4b36c983a7beb63b95cf922957378d64f
[I 21:18:21.007 NotebookApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
[C 21:18:21.087 NotebookApp]

    To access the notebook, open this file in a browser:
    Or copy and paste one of these URLs:
[I 21:18:33.777 NotebookApp] Creating new notebook in
[I 21:18:35.512 NotebookApp] Kernel started: ab30c1b3-55d2-45e9-9f01-b4bfc077bb01
[I 21:18:36.883 NotebookApp] Adapting to protocol v5.1 for kernel ab30c1b3-55d2-45e9-9f01-b4bfc077bb01
c:\users\xxxx\appdata\local\programs\python\python37-32\lib\site-packages\notebook\base\zmqhandlers.py:284: RuntimeWarning: coroutine 'WebSocketHandler.get' was never awaited
  super(AuthenticatedZMQStreamHandler, self).get(*args, **kwargs)
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
[W 21:18:37.912 NotebookApp] Replacing stale connection: ab30c1b3-55d2-45e9-9f01-b4bfc077bb01:397de5420f9e4bce82d95967bf5fa1eb
[W 21:19:14.026 NotebookApp] 404 GET /static/components/preact/preact.min.js.map (::1) 18.98ms referer=None
[W 21:19:14.040 NotebookApp] 404 GET /static/components/preact-compat/preact-compat.min.js.map (::1) 6.49ms referer=None
[W 21:19:14.050 NotebookApp] 404 GET /static/components/proptypes/index.js.map (::1) 15.99ms referer=None
[I 21:20:36.067 NotebookApp] Saving file at /Untitled1.ipynb
[W 21:22:40.052 NotebookApp] Replacing stale connection: ab30c1b3-55d2-45e9-9f01-b4bfc077bb01:397de5420f9e4bce82d95967bf5fa1eb

I have tried all Chrome, Firefox and Microsoft Edge with no success. I even tried through PyCharm.

I then looked at the js console of the browser and seems that the problem is a failure in the websocket connection:

menubar.js:277 actions jupyter-notebook:find-and-replace does not exist, still binding it in case it will be defined later...
MenuBar.bind_events @ menubar.js:277
utils.js:60 load_extensions Arguments(1)
bidi.js:19 Loaded moment locale en
utils.js:37 Loading extension: jupyter-js-widgets/extension
session.js:54 Session: kernel_created (003a523b-3d40-41ac-b65e-154db92d5528)
kernel.js:459 Starting WebSockets: ws://localhost:8888/api/kernels/f8809bf9-988f-4666-b183-e01bed63fa76
kernel.js:461 WebSocket connection to 'ws://localhost:8888/api/kernels/f8809bf9-988f-4666-b183-e01bed63fa76/channels?session_id=e49c21f065e64e7a89847a0859d689dd' failed: Error during WebSocket handshake: Unexpected response code: 200
Kernel.start_channels @ kernel.js:461
kernel.js:103 Kernel: kernel_disconnected (f8809bf9-988f-4666-b183-e01bed63fa76)
kernel.js:544 WebSocket connection failed:  ws://localhost:8888/api/kernels/f8809bf9-988f-4666-b183-e01bed63fa76 true
kernel.js:562 Connection lost, reconnecting in 1 seconds.
kernel.js:103 Kernel: kernel_reconnecting (f8809bf9-988f-4666-b183-e01bed63fa76)
kernel.js:459 Starting WebSockets: ws://localhost:8888/api/kernels/f8809bf9-988f-4666-b183-e01bed63fa76

What could be the reason here?

I had the same problem, and following this vvk2001github https://github.com/jupyter/notebook/issues/4399 fixed it for me.

Uninstall tornado 6 and reinstall tornado 5.

sudo pip3 uninstall tornado
sudo pip3 install tornado==5.1.1

@kaleo gave this answer as well to similar question from @Jason Nichols

The problem was that there was an incompatibility between the packages tornado 6 and notebook 5.7.4. notebook was updated on March 6th 2019 in order to solve this issue. Right now, the best way to solve it is the following:

Using pip

$ pip install -U jupyter

Using pipenv

$ pipenv update jupyter

The Jupyter Notebook 5.7.5 update should resolve issues with tornado 6. See the tweeted announcement and github release tag by the Jupyter team.

As of 03-06-19, both the docs and conda distribution are pending changes. You may however update using pip.

From the docs, upgrade with:

> pip install notebook --upgrade

Verify version:

> pip --version

Don’t downgrade tornado. Instead, force-reinstall jupyter.

pip3 install --force-reinstall --upgrade jupyter

Ref: https://github.com/jupyter/notebook/issues/4439

Reset your jupyter notebook configuration file by use this below command to reset default config of jupyter_notebook_config.py

jupyter notebook --generate-config

I have tried downgraded tornado and notebook it seems to be not working and unable to run cells on notebook

Found this work-around: Try downgrading Pyzmq.

pip install Pyzmq==19.0.2.

This problem occurs when you install TensorFlow.js.

When you install TensorFlow.js, it also installs prompt-toolkit-1.0.14, which causes the problem.

After installing TensorFlow.js, you just have to install prompt-toolkit-3.0.5 to solve this problem by running the following code in your command prompt:

pip install prompt-toolkit==3.0.5