Hold down the (command) key+ P+R immediately at start up all together at once and hold them down until your Mac restarts and chimes 3 times counting the start up.
DON'T LET IT CHIME / RESTART MORE THEN 3 TIMES!!!!!!!!
That clears the PRAM and that may help. Otherwise do all the steps Quad5Ny mentioned again and see if that does anything. If all of this fails something else is wrong could be RAM, could be one of many parts of the hard drive, logic board. So hard to tell. Hope this helps, otherwise I'd be happy to help you further if you wanna drop it by for service.