Power on with no start up chime by iMAC G5

I use in addition to my relatively modern iMac that I am typing this up on, a 20" iMac 1.8 GHz G5 (2005, separate iSight) with 2 GB RAM, running Mac OS X Leopard (10.5.8). The system has failed me for the first time today, unexpectedly. It powers on, I hear a brief, subdued sound like a matchstick striking a friction surface, followed very briefly by a sound like the fans accelerating and then slowing back down, and no screen activity or start up chime. About 45 seconds later, the fans seem to rev up to full speed and stay that way.

I am unable to insert a CD (the retail Leopard OS X installation disc, the machine started with Panther and migrated through Tiger to Leopard) and there appears to be no response to a PMU/SMC reset. Apple service where I am (Bangalore, India) tells me that with my machine being obsolete, they're not in a position to help. I would like very much to salvage the system, though I don't really need to, and seek any advice to try and troubleshoot the problem.