My Mac Pro will not start up

power on? have you checked the power cables, plugged into a different outlet? reset the SMC and NVRAM?

Reset the System Management Controller (SMC) on your Mac - Apple Support

Will it no load OS X? have you tried safe mode

Try safe mode if your Mac doesn't finish starting up - Apple Support

what are the symptoms and what have you tried? Remove some of the guess work for us and we can provide a quicker answer.

you wrote Mac Pro, your sig says MacBook Pro, these are two very different computers when they won't power on so which system is it?

We're going to need a little more information. Some questions follow...

Both "Mac Pro" and "MacBook Pro" are listed — which Mac Pro or MacBook Pro model is being used, if you happen to know that? Old or new Mac Pro, or an approximate year for the MacBook Pro...

Is the system under warranty?

Maybe If your Mac won't turn on - Apple Support applies?

Do you get the usual startup chime? (That's the result of a hardware test.)

If it won't boot, what icon or what display do you see around the time of the failure?

Have you tried booting from some other disk?

Here's an overview of how to troubleshoot.