How to troubleshoot macbook's failing parts
Try resetting the SMC and resetting the PRAM.

It's also possible that your hard drive is failing in such a way that the computer can't boot. I haven't seen that often but it can happen (if you hear weird clicking sounds on boot, that increases the likelihood of this being the case).

It's pretty easy to take the hard drive out of your computer (all you need is a a phillips-head #00), it may be worth removing the hard drive, then trying to boot to an external disk.

MacBook Pro take-apart guides.