Mac Pro 2009/10 Doesn't boot after weekend, LED shows for 2 secs then nothing!
We just managed to figure this out. It was a faulty RAM board/riser. We found the DIAG switch and pressed it with a pencil, all the lights for the PSU checked out properly. So we went back to thinking it must be the RAM again. One riser had 10GB on the other had 2GB, luckily the 2GB one appears faulty. We removed this riser (although we'd already done this!) and the machine booted up fine!

We then reseated the sticks in different slots on the faulty riser, reseated the riser and the machine booted, but couldn't find the RAM. So for now we've ditched that riser and are letting the machine run with 10GB instead of 12GB.

Hope this helps.