PSU Dead?

Yesterday I was playing a game of starcraft 2 and there was a thunderstorm outside but I didn't mind it at first... The power went out for a second then turned back on...but my computer wouldn't turn on :0 So I looked around the surge protector to see if there were any burn marks or something that smelt weird but it was all clean. So I tried plugging the psu into multiple outlets and the only thing I got was the power button turned on for half a second then immediately shut back off. So I looked inside the computer for anything weird but I couldn't find or smell anything wrong. I took out the PSU and put in a older one which I had replaced and sure enough it turned the computer on! So now I'm stuck with a low end graphics card compared to the good one I had bought with the new psu. The PSU was 100watts more than I needed just to be safe. So does this mean that its dead? I've never had one die before because this is the first one I've ever bought. It is a corsair builder series cx500. After all this I got a random blue screen and then the computer restarted with no other problems...