Either😥

security settings OR an ad blocker.

You would get the best advice if you could describe exactly what you do see, the browser you are using and what security package your use and ad blocker apps/add-ins. Also does it work on another browser?

It might help to know more about your system and browser and what you've done so far to try and get it to work.

- Windows?
- Using Chrome? Firefox?
- Reboot?
- Clear cookies?
- Youtube and DU videos ... and Netflix or other streaming services?
- Do you have any videos stored on your computer (from your phone/cameras), if so, do they still play?
- Checked for virus and malware?

Hmm, are you using WiFi or getting Internet over your cellular network connection?? I'm wondering if you have some kind of cellular data plan with your provider that limits video if you've exceeded some usage limit?

Anyway, in no particular order, some things I'd try (one at a time and see if videos work afterwards):

0) Update your iPhone to latest iOS available for your device
1) See if it fails when you're using WiFi and also when you're using Internet over your cellular connection
2) Close and reopen Safari (https://support.apple.com/en-us/HT201330)
3) Power off WiFi and turn it back on
4) Put the iPhone into Airplane mode and take it back out
5) Power off/on the iPhone (not just lock the screen, fully power off and on)
6) Take it to an Apple retail store and have them check it

Good luck