Hmm, could be a few things but if it's anything but a slim the blu-ray drive might have failed (sounds like this is it, try to Google it). While not as infamous as the RROD for the Xbox they have had a significant failure rate on the first drives.
I had it happen to me and spent $120 to have it repaired at a small electronics shop (slim wasn't out yet). I think it was $140 to have Sony fix it, and I didn't want them to profit to fix something they didn't build properly (the parts cost about $40 I think).
I would have done it myself but was having a problem finding a blu-ray drive on ebay since the above mentioned electronics shops would snatch them all up so they could have their techs repair it for some easy money.