When I attempted to fire up the MacPro (Mountain Lion) after some months abroad the fan started up but the machine then shut down after a mere six seconds. Someone did use it during my absence, and I am now using her Windows machine; Help, somebody!
See if it will start in Safe mode - hold down the Shift key while it starts. It will be much slower than normal but you may be able to diagnose from there.
If you have the original discs, you could start from them and run the Apple Hardware Test
Eric Jervis 