[blinki bill]

the problem you probably have is that your ram memory goes off; when you start the computer it gives sound if there is problem with the ram and doesn't starts at all but if it's working then and stops working later during the ram check the computer freezes at the system logo screen.
If you remove the ram and put it back usually makes the computer start normally but it's going off slowly and at one point nothing will help
If you have the problem again you can try with replacing the ram