Lazer for ps3 HELP!

So my ps3 has had some problems it will not read discs. the laser does not work however its brand new and tries to work. when i firt turn my ps3 on and take the disc out while the ps3 still thinks its in there the laser comes on and the dvd drive goes back down with the laser on and tries to look for a disc. but after that if i put it back together and put the disc in it the laser stops working. why is it doing that?