My MacBook Pro won't start. I get a grey screen when I turn it on. Please help

Hello.

I own a MacBook Pro 13" Retina, it's barely six months old. I bought it in January. I've been using it for most of the day today so I shut it down to take a break about 4 hours ago. but when I turned it back on I cdnt get past the the grey screen. I don't know what to do next, could somebody please help me. It couldn't be "too much" data, I've barely downloaded anything on it, but pictures and music. So I'm short of solutions. Virus? Battery? Someone please help.