Why won’t my iPhone play videos?

Water damage can cause all sorts of random issue, but this sounds more like a software problem.

I would backup the phone via itunes and restore it afterwards.

Try to record a video and play it before reinstalling your back up. If it still does not work, then there is a hardware issue in the board, but this problem sounds like a software issue.

I hope this helps

Since you've already tried reinstalling apps and rebooting, it might be a deeper issue. Water damage could definitely be a factor, especially if the internal components responsible for video playback (like storage or processing chips) were affected. You might also want to check if the issue happens with all videos or just specific ones—if only some are affected, they might be corrupted. In that case, you can try repairing them using the methods outlined here: https://news.macgasm.net/tips/fix-corrup.... If none of these solutions work, it may be worth running an Apple diagnostic or getting your device checked by a professional.