When your Mac consistently hangs at the loading screen you described (Apple logo with spinning gear) the best advice I can think of is to reboot using verbose mode.
To boot into verbose mode:
Power the Mac on and hold ⌘+V until you see a black screen with lines of text.
Verbose mode displays what is happening while the Mac boots, the stuff that is normally hidden behind that gray screen with the Apple logo and spinning gear.
Try powering it off (hold the power button for 5-10 seconds) then powering back on and entering verbose mode using the method described above. Give it a minute and check to see where it's hanging. It might not give you a clear cut answer but it can surely provide some clues.
In my experience this often comes back to a failing/failed hard drive. This is often reinforced when I see entries for "Disk I/O read/write error" or similar in verbose mode.
Assuming you don't see Disk I/O errors you might also try resetting the PRAM and/or SMC.
To reset the PRAM:
Power on your Mac while holding ⌘+option+P+R until you hear the Mac chime twice, then release the keys and let it boot normally.
The SMC reset is somewhat model dependent.
For Macs with a removable battery: Remove the battery and disconnect the power cord, hold the power button for 5 seconds, reinstall the battery, reconnect the power cord and power on.
For Macs without a removable battery:
Connect your Mac to it's MagSafe adapter, on the left side of the Mac press and hold Shift+control+option and the power button for 1 second then release all at the same time. Without holding any keys press the power button again to power on the Mac.
Hopefully that helps!