Were your videos and photos stored on the SD Card? If so, and you can no longer access your card, then that would be the reason why you can't see them. If your SD Card failed, then your photos and videos (and everything else on the card) could be lost.
Shut your phone down (that will make sure that everything is closed/unmounted properly), take the card out and put it in your computer and see if your PC can read it. If it can, copy EVERYTHING off of it as a backup. Put it back in your phone and start your phone. See if you can read the files now. It could be a glitch in Android that unmounted the card on you, or your card could be going bad.