Why is my computer hanging at startup?

This started a few days ago. My macbooks started freezing during YouTube videos or other video ads and such. Resting it normally would fix it. Then it progressively got worse to where it would only open in safe mode. Now it won't boot up at all. All lights are functioning normally (I.e breathing light, background, keyboard, Apple logo on back.) I have reset the PRAM yet I don't hear a chime it just reboot over and over again. It hangs at the opening Apple logo with the spinning wheel below it. After 15 seconds it just stops spinning. HELP!!!!