in response to 'expandables' relply - just use a regular PC is not the answer and what an idiot for saying so - People like this are so off-putting to someone who wants to learn or solve a problem. Not everyone has a 'Regular PC' to just use as-well as their Pi, and anyway, instead of this easy way of avoiding the problem this forum is designed to help people overcome problems and find solutions. If everyone had your attitude No-One would ever progress or learn anything.
In response to the original post - I have the same issue Raspbian Jessie won't play Youtube videos - although you say yours did originally? my Pi won't play youtube on Raspbian but will on OSMC so it is software related. I wonder if there are additional drivers needed or software. I tried doing it through the Chromium browser and no luck, BUT, wheb I connect my LCD touchscreen (running a customised Raspbian that comes with the screen) I can play Youtube vids at a low quality.
I don't have a 'regular PC' to just use instead and I want to play Youtube tutorials while testing things and learning on Python. It seems that the Pi itself is mre than capable of video playback but the software (Raspbian) prevents it from doing so. Even on OSMC youtube wouldnt run - it freezes, but all the other video / TV channels played.
If I was on a regular pc the fix would be to install the latest flash player - however that is for Windows