This is a tough one to answer since there are more variables at play then just the iPhone and iOS8. Which version of the iPhone? Do you have a Mac or a PC? The videos that are no longer playing, do they play on your computer? How did you acquire the videos (purchased from iTunes, burned them from a DVD, recorded them on the iPhone?
Without knowing the answers to the above I would:
1) Resync the phone with iTunes,
2) if that fails, try restoring your phone back to factory default...read this for assistance. The retransfer your movies to your phone.
3) As a last resort, if you have a MAC, open one of the videos in Quicktime, click file....>Export....Export for iPhone. This will reencode the video using an updated CODEC.