there are couple of ways to do so. my proposal would be to either:

1) delete contents of
/home/user/.thumbnails/video-grid (if it's just videos you want to recreate) and reboot

2) move all movies to a different location -> launch video player -> wait until it shows no content -> close it -> and than move them back to the previous location. now they should appear with rebuilt thumbnails

same procedure for any other thumbnails, just search for different subfolders.
once for a while I'm purging all .thumbnails subfolders as this catalogue can get huge over time.