What to do if your OS X system fails to boot, being stuck at gray screen with the moving-wheel?
The system boots from recovery partition, it does check the partition but it will not boot.
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The OS X boot process is chock full of little tasks in parallel and sequence, but it boils down to three major parts:
Since you haven't gotten to the blue screen - it's not anything in your user account as the system itself hasn't booted. Rather than guess what exactly has caused it, I'll link to the two articles that Apple has on getting to the nub of startup issues.
I am including both since you may have to solve user issues if the cause of your boot is hard drive corruption or major hardware issues. Hopefully it's just one thing in the system startup.
If you don't have time - start with a Safe Boot (hold shift key down) to see if you need to get your recovery disk or another bootable system. If you have a backup - you can jump to re-installing the system since that should re-write everything needed to get to a blue screen. Do run a safe boot before or after to rule out incompatible non-essential system level startup tasks - it could be as simple as one launchd problem during the system start process. The system logs will likely point you to what is going on when it pauses.
Good Luck and let us know what you find out!
The first thing I would do would be to use target mode to see if the internal drive mounts and if so, back up the data. Then I would run Disk Utility on it, either while it's connected via target mode or by booting from the OS DVD. Any further steps would depend on the results of these.