It seems to me as though the installation has messed up somehow. Try running:
# For Python 2
pip install --upgrade --force-reinstall --no-cache-dir jupyter
# For Python 3
pip3 install --upgrade --force-reinstall --no-cache-dir jupyter
This should reinstall everything from PyPi. This should solve the problem as I think running pip install "ipython[notebook]"
messed things up.