First I would recommend quitting all your apps from app switcher and restarting your device.
Next, try launching iMovie.
Does iMovie Launch, and does the project load successfully?
Or does iMovie Launch but loading the project crashes?
In any case, since you have a long project pinch the timeline to Zoom-out, that should reduce the number of thumbnails which are being displayed and hopefully lower your memory usage.
Additionally a new version of iOS is out and the same goes for iMovie for iOS.