Mac Pro - boot up problem

Hi, my G5 desktop has recently developed a start up problem and i was wondering if anyone could offer me advice?

When i boot up occasionally the OSX start chime will sound then all i get is a grey screen.

Mostly there is no chime and the screen doesnt kick in at all.

There's a clicking sound which seems to be coming from the cd/dvd drive, (which wont open) so i cant attempt to start up from the install disc.

The last thing that happened before all this was that the cursor froze, after which i attempted to reboot....any thoughts?


Many Thanks J