Hello and welcome to my User Tip
The gray screen issue at boot can be caused by many things.
1: If you just did a Software Update, the firmware progress bar might be updating, let it complete and if it's stuck for more than 10 minutes, hold the power button down and force a hardware (emergency use only) reboot.
2: If you boot to a Gray Screen with icons of drives to boot from, that's Startup Manager and either your option/alt key is stuck or your PRAM needs to be reset and then head to System Preferences > Startup Disk and reset it. (along with your date and time, other settings) See #2 PRAM reset here: ..Step by Step to fix your Mac
3: If that's not it nor doesn't fix the issue, reboot again holding the Shift Key down (wired or built in keyboard, PC one will work), this enters "Safe Mode" and disables certain things like third party at boot kernel extension files that get installed by some third party software. Go around and update all your third party software or uninstall it using the developers uninstall methods (never use App Deletes type programs!)
If you don't have a recent copy of your personal files off the machine, it's advised to do so now to a external storage drive (do no use or trust only TimeMachine for this)
Reboot again normally to exit Safe Mode.
3: If Safe Mode fix doesn't work, it's likely a corrupted OS X, run Disk Utility #5 to check for drive issues (and if it exists) and it's possible to reinstall OS X over itself #8 in the Step by Step's to fix it. Software Update fully immediatly afterwards or else your going to have issues with your files and security.
When you do this, any third party kext file in OS X itself is removed so you'll have to reinstall third party software that no longer works. But of course do so slowly as to see when the problem presents itself again, then you know and can contact the developer and/or uninstall the software.
Snow Leopard users might have a extra problem, some developers may no longer issue 10.6 versions of their software for reinstall, so if you didn't save the 10.6 installer package when you first installed the software, you need to take more manual steps to remove these third party at boot kext files before you do a #8 Reinstall OS X.
4: Sometimes it can't be fixed as there is a drive or file structure corruption, and the progress bar your seeing is the fsck drive repair attempting to fix it which it can't, so it freezes and your stuck like that.
Data recovery steps need to be taken first if you don't have at least two copies of your recent personal data file off the machine. Do not trust only the TimeMachine copy, it might also be corrupted and fail to properly restore.
If you have drive corruption and the fsck boot drive check can't repair it.
If you have a copy of your recent data off the machine,
Then option/alt key boot from the 10.6 disks, or command r boot into 10.7/10.8 RecoveryHD and security > zero erase (or moving slider one spot to the right) your MacintoshHD and restore from your TimeMachine backup or install OS X again and restore from TM or other backups.
If you need the entire drive erased, the use 10.6 disks to do that or hold command option r on a Ethernet connection to a router (or near your own) and use that Disk Utlitiy to erase the entire drive (moving slider one spot to the right), then reinstall OS X (whatever version) and then log in a AppStore upgrade, then restore from TimeMachine or other backups.
If you have a bootable clone with Carbon Copy Cloner or Superdupes, then merely hold option/alt and boot off that and grab files and reverse clone.
If you have no backups and need to grab your data off the non-booting internal drive, here are some options.
If I'm not making any sense, then seek the services of a local PC/Mac tech that does data recovery and will restore your software, else any hardware repairs are better off left for Apple to do, especially if your under AppleCare or warranty as it's no cost to you, however Apple doesn't do data recovery.
Once the data is recovered, then more stringent methods can be employed and then restored from the TimeMachine or bootable clone backup.
Didn't make a backup yet? You will now.
*work in progress*