Unfortunately, since the last OS update for the Mac (Monterey), QuickLook has become especially unstable, especially on my M1 Mac Mini. It doesn’t cause crashes or anything, it simply stops working; especially with video files. However there is a simple solution to the problem. From the Terminal, just run the command (you do not need
killall -HUP QuickLookUIService
This will restart just the QuickLook service, and it should start working for you again. Until it doesn’t.