artifacts on screen

Ok what happened was, I was playing counter strike on my notebook when all of a suddent the screen started getting messy and in a while it was all garbage on the screen and I could make up nothing. I closed my notebook thinking something is wrong with my video card now. I turned it on and I was still getting some artifacts and distortions of the graphics. But I restarted it 3 or 4 times and till now everything seems to be working fine and I didn't see any distortion of the graphics yet. Could anyone explain what could be wrong? As far as I know these things could happen if you overclock ur graphics card but I did nothing like that.  I have a ATI Radeon 64 MB  btw....  can anyone help me plz ?