Q: Mac pro will not restart.

You are still using 10.6.8, just to confirm, so you don't have Recovery Mode for one thing. But do you have a backup that is bootable? make one now before you do more. And try repairing the drive as well as doing a Safe Boot.

You may need to do a clean install to another drive. Apparently you have another Mac or something to post and get online so you could also use Target Disk Mode to access the system in trouble.

Update with what from what? what other internal drives are there?

http://macperformanceguide.com/Mac-HowToClone.html

http://www.macupdate.com/app/mac/7032/carbon-copy-cloner

Using Cloning as a Backup Strategy

Create an OS X Lion Install disc

How to create an OS X Lion installation disc MacFixIt

Migration Assistant Update for Mac OS X Snow Leopard

http://www.apple.com/support/lion/installrecovery/

http://reviews.cnet.com/8301-13727_7-20080989-263/how-to-create-an-os-x-lion-ins tallation-disc

http://www.coolestguyplanettech.com/how-to-make-a-bootable-osx-10-8-mountain-lio n-disc-or-drive-from-the-downloaded-mountain-lion-app/

General Mac maintenance: Tips to keep your Mac in top form

General purpose Mac troubleshooting guide: Isolating issues in Mac OS X

Creating a temporary account to isolate user-specific problems: Isolating an issue by using another user account

To resolve startup issues and perform disk maintenance, use Disk Utility.

Safe Mode or "Safe Boot" is a troubleshooting mode that bypasses all third party system extensions and loads only required system components. Read about it: Starting up in Safe Mode