Hi
Have you looked in the "video" section in the left hand side bar.
Also look in the "imports" folder.
If they are not there, have a look in "recently deleted"
If not in any of those places, is it possible you have imported them into a different library from the one you have open now? Hold down the option key while starting photos. In the resulting pane, is more than one library listed. If so, open each in turn to see if your videos are in them. (Before doing this make sure any external drives are connected and mounted, if there is any chance you have a photos library on an external drive)