My macbook pro will not get past the gray screen with the apple logo and a loading bar.

My macbook pro will not get past the grey screen with the apple logo after attempting to load for hours. I have already tried resetting the NVRAM/PRAM and a couple other tricks and no luck. Anyone out there have suggestions for my problem? Help would be much appreciated since i use my computer for everything at school and i am in desperate need to get this baby working again.