If it boots in safe mode reinstall the OS and try a different version of OS X, like if your on Mavericks try Yosemite instead. If it still does t boot replace the hard drive. And if it still doesn't boot with a new hd I got good news and bad news, the good news is you probably need a new cable that goes from the hd the mother board and the bad news is it could also mean your mother board is done.