There could be many reasons why Jupyter Notebook may not open, we take a look at the top 5 reasons.
Reason 1: You are using Incorrect Command:
If you are having trouble running the command in CMD or Terminal then you can try one of the below commands as per your setup and see if it works. I have tried to collect as many ways as I know if I have missed any out, please add it to the comment so others may find it helpful.
Windows Command Prompt (CMD):
jupyter notebook
If Using Anaconda Prompt (Conda):
jupyter notebook
General: Using Python3
jupyter notebook
Terminal on macOS/Linux/Unix:
python3 -m notebook
Using Python:
python -m notebook
If you still are having issues opening Jupyter, then you can try to install a package called nbopen, it is a really cool module that will help you open the Notebook with just a single command
pip install nbopen
or,
pip3 install nbopen
Now all you need to do is run nbopen command in the Terminal/CMD.
Reason 2: You do not have Jupyter Notebook Installed
One of the most common reasons why Jupyter Notebook will not start is you have activated a Python/Conda environment that does not have notebook installed.
Example: % source activate
(virtualenv) Code2care@Mac % python3 -m notebook
/Users/c2cdev/Desktop/virtualenv/bin/python3: No module named notebook
Now when I deactivate the virtual environment, the same command works!
Reason 3: There is a Port Conflict!
At times there could be some application using port 8888, in such case if you make use of the --port option to choose another port.
Example:jupyter notebook --port 8899
Reason 4: Firewall or Antivirus Blocking Notebook
There is a possibility that your firewall or antivirus software might be blocking the connection to the Jupyter Notebook server. To fix this, make sure to add the Jupyter Notebook as an exception rule.
Reason 5: Dependencies and Extensions Issue
There can be incorrect or conflicting dependencies or notebook extensions which is causing Jupyter Notebook startup failures.
You can disable the extensions and try if it works.
jupyter notebook --no-browser

Facing issues? Have Questions? Post them here! I am happy to answer!
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