Monitor remains grey after Apple logo during boot on Mac Pro
This happened earlier this year to a friend. The cause was disk
    directory damage. Upon start (during the Gray Screen of Doom) the
    Mac was trying to repair the damage automatically, but failing.
    Here's how I talked her through it.


Power down the Mac.
Boot in Safe Mode (press Power button, listen for startup
tone, immediately press and hold Shift, release Shift when you see
Apple logo).
Let the computer boot in Safe Mode. It may take longer  than
normal.
If the computer eventually reaches the desktop, simply reboot
    normally and see if that fixed your problem. This is the point at which this procedure worked out for my friend.
If the computer is still having problems, boot from a Mac OS X
    Install DVD (insert DVD, power down, restart hiding down C key).
When you reach the Installer screen, use the Utilities menu to
run
    Disk Utility, then choose Repair Disk under the First Aid tab (if the hard disk doesn't show up in the sidebar in Disk Utility, the disk is likely irretrievably broken).


If these steps don't work, I'd suggest that you either a) Wipe the disk and do a full restore from your most recent backup; b) invest in a heavy-duty disk utility like Disk Warrior; c) take the computer to the nearest Apple Store for support help.