With the new version of ipywidgets, I ran into this problem, but then I found on their github readme that you now need to follow
pip install ipywidgets
with
jupyter nbextension enable --py widgetsnbextension
Note: To those using virtual environments (including conda environments) the recommended way to activate the extension is to run
jupyter nbextension enable --py --sys-prefix widgetsnbextension
That cleared the problem up for me.