Hmm... try doing this:
1) Force your Mac to shut down by pressing and holding the power button for about 5 sec.
2) Start in Safe Mode. Press the power button to turn it on, and before you hear the Startup "Dummmmm" chime, hold down the Shift key. For a few sec, you won't see the Apple logo. Keep pressing the Shift key until you see it. If you press it after the Apple logo appears, start from Step 1 again.
3) This time, your Mac should take longer to boot. (Like a minute or 2 longer as it is going thru the necessary files to boot from, and not just everything like in Normal Startup mode; which caused it to hang)
4) Does your Mac boot sucessfully to the Login Screen or Desktop? The login Screen should show SAFE MODE in red at the top. The Desktop should have Sound disabled and seems a lot slower. If you see either of these, backup ALL your files (if you have not already done so) on to an External HDD. Create a new User Account in System Preferences>Accounts with about the same info as your current account (The name will have to be different though, if your previous account was John, this time, add your last name as in John Appleseed.) Make sure it is an Admin account. Then, login into the new account and delete your previous account. Copy all your files over to your new account and restart your Mac correctly by going to Apple Menu>Restart.
5) This time, don't press anything and let it boot normally. Does your Mac boot correctly? I hope that helps you a little. Get back to me if it does not work! 😉