Won't power on?!

It would appear that the battery may be completely drained and the charger is not charging it. First try doing an SMC reset: http://support.apple.com/kb/HT3964

If it doesn't help, try using a different charger of the same capacity (85W for 15" & 17", 65W for 13", 45W for Airs, stamped on the case). If it remains the same, will need to get it checked at an Apple Shop or a knowledgeable service provider.

Here is an additional Apple support article that may be of use:

http://support.apple.com/kb/TS1367

Ciao.

Thanks I switched power cords and that powered up the Mac Pro.....:)