Welcome to Apple Support Communities.
I understand that you are having an issue playing videos on your iPhone. I use my iPhone a lot and know how important it is to have your device working the way you want. I'm here to help.
Please force close all the apps on the device, and restart the iPhone. Once this has been completed, please retest.
Force an app to close on your iPhone, iPad, or iPod touch
Restart your iPhone, iPad, or iPod touch
If needed, you can follow the remaining steps in the article listed below:
If an app unexpectedly quits, stops responding, or won’t open
Please let us know if you are still having issues.
Have a great day!