Failure to boot up (by 2 users)

I failed to re-insert a 256mb PC2100 DDR Dimm into its slot correctly. The computer booted up the first time but with obvious consequences. I presumed I had burnt out the slot and moved the memory to another slot. The computer booted up fine the first time but then refused to boot up again. Moved the memory to the third and final slot only to have the same rigmoral. I have replaced the memory but the computer booted up the first time and now won't boot up at all. Same situation every time - can hear it powering up but it never actually boots up with nothing on screen. Wondering if I've burnt the processer out or what else would allow you to boot up first time but not second time?