This isn't the first time it's happened but now it doesn't seem it's going to go away. I thought the ps3 couldn't read any discs until my nephew put in a dvd and it played fine. I thought it was fixed and put in a ps3 disc, and nothing loaded
its a seperate laser and its known for having a short life <= 1 year usually. in some ps3s the actual diode can be replaced but these days its cheaper just to replace the whole laser head inside the drive. the hard part is knowing which laser to buy. back on topic - if it is only reading dvds and cds and not blu ray games or movies then its almost certainly a new laser required for you.
to buy you need to know what you have. theres 2 different types of laser modules for phat ps3s - one has a single lense and the other has 2 lenses and they are not compatible. if its a slim i dont know the laser configuration for them. whether its a phat ps3 or a slim you need to open your ps3 and remove the bluray drive and open it to see what kind of laser is inside it then simply buy one of those.
youtube is your friend if you need help dismantling your ps3 and blu ray drive. one small tip - dont break the plastic clips.