I update my mac pro 2 days ago and it will not start up I have a gray screen and 1/2 bar down load showing what up with that

- Try resetting the SMC and NVRAM/PRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

About NVRAM and PRAM

- Try starting in Safe Mode

OS X: What is Safe Boot, Safe Mode?

- Boot to Recovery and repair the startup disk

OS X: About OS X Recovery - Apple Support

- If it is repairable reinstall the OSX

How to reinstall OS X on your Mac - Apple Support

- If you do not have a backup use disk utility to restore the internal disk to an external disk so you can try to recover data.

Then format the startup disk and do a fresh install of OSX

At power-on, your mac starts the fans, lights up the screen, and chimes. That means it has passed the Power On Self Test. Next, it gets a Blob of software (containing the dark gray Apple) off the boot drive using a very primitive method (the file system is not loaded yet). Then it does a lot of initialization, and eventually initializes the file system. Next it tries to get the File System to Mount the Boot Drive so it can continue loading.

If the drive can not be mounted, you get a progress bar and spend five minutes doing a disk repair. When completed:

--if the drive can be mounted, loading continues.

--if the drive cannot be mounted due to too much Directory damage, your Mac will shut down, since it cannot proceed.

You will need to use Recovery mode to run Disk Utility and run repair disk repeatedly until it comes clean or gets stuck. If it is stuck, Your drive cannot be written to.

Buy a new Drive and an external enclosure and Install mac OS X anew on it. Then use that Mac OS X to rescue anything still needed from the old rive, with Utilities like Data Rescue. Once you are satisfied, you can swap drives around to get the new drive where you want it.

Boot from your backup drive, erase this drive, then clone your backup to it. If you don't have a backup, then you will need to try this instead:

1. Restart the computer. Immediately after the chime hold down the Command and R keys until the Utility Menu appears.
2. When the Utility Menu appears select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the the side list.
4. Click on the First Aid tab in Disk Utility's main window, then click on Repair Disk button.
5. If Disk Utility reports errors that have been fixed, re-run Repair Disk until no errors are reported.
6. If there are no errors click on the Repair Permissions button.
7. Wait for the process to finish, then quit Disk Utility.
8. Select Reinstall OS X and click on the Continue button.