Why is my video card heating up so fast?

I have a ATI Radeon 5770 in my PC right now and I've had it for about a year. Recently, it has been heating up way faster than usual and the fan doesn't seem to spin up to cool it down. I'm not sure what it has peaked at but i saw it was at 75 C. It's not cooling down and i keep getting black screens, im not sure if the black screens are coming from the video card being hot, but in any case, can anyone help me?|||Did you blow the dust out of the cooling fins? 3 times a year the dust needs to be blown out of the pc.