Welcome to Android Central! Which phone? Are the videos saved to Internal Storage or a microSD card? If the latter, the card might be corrupt. Unmount the card (in
Settings>Storage), remove it, and insert it into your computer.
Back up all of the data on the card to the hard drive now, and then run chkdsk to look for bad sectors.
http://forums.androidcentral.com/am...guide-using-chkdsk-fix-corrupted-sd-card.html.
If the videos are saved to Internal Storage, connect the phone via USB to your computer, and use Windows Explorer to copy the video files to the hard drive--can you view the videos on the computer? (They should be in the /DCIM/Camera directory on the phone.)
Remember that flash memory does not last forever--it can fail, especially SD cards. You should
always back up important data to your computer hard drive and/or the cloud (like to your Google Photos in the cloud).