It seems to me as if your installation has messed up completely. Try running below commands :
# For the Python 2
pip install --upgrade --force-reinstall --no-cache-dir jupyter
# For the Python 3
pip3 install --upgrade --force-reinstall --no-cache-dir jupyter
Above commands should reinstall everything from the PyPi. This should also solve your problem as I guess by running
pip install "ipython[notebook]" messed things up for you.
As both your pip and pip 3.6 were installed and the pip command was failing:
pip install --upgrade --force-reinstall jupyter
So I tried to use the pip3.6 command as below:
pip3.6 install --upgrade --force-reinstall jupyter
And the above pip3.6 command just worked for me. Running the
jupyter notebook also worked after the installation.
Hope my solution helps you in resolving all your issues.