|... adding to the above. You can check which version of Adobe Flash you have installed and a list of latest versions from here:|
If you use more than one browser then check it from all of them.
Seems my life is incomplete LOL. I use YouTube a lot and have never had need to install HTML 5, its always been Flash.
It seems strange that it suddenly stopped working. Could be internet difficulties, so maybe it will suddenly start working again. I assume you tried restarting the computer. There is a vague possibility that your server is doing some limiting.
Always pop back and let us know the outcome - thanks