Help! Bike will rev but wont move!
 

I was riding my '99 zx9R up a hill and I lost all power. I pulled over and took a look but i couldn't see anything. It'll go into first but when I give it gas it just revs. If I turn of the bike off and put it in gear it still rolls. I was looking it up and i couldn't find anything about it. SOMEONE PLEASE HELP!