First I would try moving them back to the phones internal storage and see if they show up fine again. If so, your card may be the problem.
If that doesn't work, you should try connecting the phone to a computer via USB cord and see if you can access the files that way.
NOTE: If the videos are not ones you recorded yourself, but are say from Netflix or other provider like that then they won't work anywhere but on the phone itself (not even the card). It is a restriction they placed on that (for obvious reasons).