OK, I just bought a second hand pismo G3 - it worked fine, I tested it first.
I opened her up to swap over the hard disk with an old one(which also previously worked fine). I put everything back together, and all that happens now when I start her up is the Hard Disk starts up. Nothing else. The screen is dead, there aren't the usual processing sounds you usually get, just a steady hard disk sound. I've triple checked everything that I tampered with - which was only the heat shield, the processor and the hard disk. All the other cables are as they should be.
Any suggestions would be most welcome.
thanks!!