Dead computer? HELP!!

[galadial]

I think I may have killed my computer. I was fiddling about replacing an IDE ribbon. After changing the ribbon, the computer would not power up. The PSU fan doesn't spin, it makes no attempt to boot, and no LEDs come on inside the case. Curiously, there is power to the monitor and printer. Does anyone have any ideas what has happened or how to troubleshoot?

I do have a second computer to savlage parts to test - so I could swap out the PSU etc. Any thoughts on how to proceed?

 

[galadial]

Thanks guys for all the great support. I eventually sorted the problem. Only when I had removed all the hardware to try and isolate the problem did see two tiny pins which had come loose from the motherboard. This was the front panel power switch. Couldn't figure out which pin was the right way round, so guessed, and everything seems to work. I'm surprised how tough computer parts are - by rights I should have shorted or fused something, but as far as I can tell, no damage done.