Ok well your best bet is to either plug that hard drive into a working mac and move stuff off of the drive onto your macs hard drive, or you can put the hard drive back into the mac it came from, hold cmd+R, then reinstall os x, you won't have to lose your files. In the cmd+r state you can also just run disk utility which will enable you to check your drive for errors, or completely format it (after you have backed up your stuff of course).
If for any reason none of that works, I would personally run the disk utility from the cmd+r mode, create a new partition on the hard drive (even if it's tiny for now it can be changed later), install a fresh copy of mac osx on that partition and then it will boot.
Also, back to square one, do you know why it's not booting? A full hard drive wouldn't stop a mac from booting, it would impair the performance, but not stop it from booting.