Hi ollie89,
Thank you for using the Apple Support Communities! I understand you are unable to play a video within your Photos app on your iPhone. I'd like to help.
Do you have iCloud Photos turned on? Set up and use iCloud Photos - Apple Support. You can confirm this under Settings > [your name] > iCloud > Photos > iCloud Photos. If so, are you signed in on any other device using the same Apple ID account? If you are, can you test to see if you are able to play the video from another device without issue, please? If you don't have another device, you can also log into the iCloud website and click Photos, to test if the video plays from there without issue.
If you can play the video from the iCloud website, or another device, I would suggest following the steps below, on your affected iPhone:
-Restart your iPhone: Restart your iPhone - Apple Support.
-Connect to a different Wi-Fi network, and see if you can load/play the video without issue: Connect iPhone to the Internet - Apple Support. This can help rule out a network-related cause.
-Consider updating to the latest supported iOS for your iPhone model: Update your iPhone, iPad, or iPod touch - Apple Support.
Keep us posted on the results, if you need continued support. Regards.