What controls and lets you play higher youtube video quality?

I used to be able to watch in 1080p and 720p youtube videos but now if i try setting it to that level, the video has a big exclamation mark in the centre and the video wont run. is that a video card issue? do i just need to update my video card drive?|||try updating flash first possibly? also what browser are you running? it is not a video card issue, though. does the video say anything but the "!"? could you upload a screenshot?