The update might have triggered this issue. You could use your Time Machine backup to revert back to your previous version of iMovie, and see if that solves it.
Here are some diagnostic procedures that you can try:
Updating
iMovie
To update/redownload iMovie 10, drag your
iMovie 10 app from the Applications folder into the trash, but do not empty the
trash. Sign in to the app store and open your purchases folder by
clicking on your name at the bottom of the app store side bar. You will see your purchases
displayed, including iMovie 10. Redownload/update iMovie from there. If not
successful, drag your present iMovie app out of the trash and return it to the Applications folder.
Deleting
Preferences
Try opening iMovie while holding down the
Option and Command keys and selecting to delete preferences in the box that
appears. iMovie will open in a new library. Reopen your old library to get back
to your projects.
New
Project
Another thing you can try is to copy your
media into a newly created project and see if that cures the problem. To do
that, do an Edit/Select All, Edit/Copy, of the media in your project and then
Edit/Paste it into the newly created project.
Safe
Mode
You could also try booting up in Safe
Mode (see Finder menu for instructions) and, after everything loads,
immediately shut down and reboot in normal mode. Then see if iMovie works
properly.
Resetting
the System Management Controller
Try resetting the System Management
Controller:
https://support.apple.com/en-us/HT201295
-- Rich