Why is my MacBook not starting up?

I’ve never had any problem like this before; I’ve had this 15-inch MacBook Pro since August 2009. Today I come home and it won’t start up. It just stays on the gray apple screen in the beginning.

However, I was away for 4 days and left the computer on stand-by during that whole time. When I came back, I opened the computer and it was working; I even went on Fluther. Then I left for a few minutes and came back and my computer had gone to sleep. When I woke it up, it asked me for my password like it always does, but when I typed it in, nothing happened. It said “checking password” for almost 5 minutes, so I just held down the power button and shut it off (which I’ve done before when it freezes).

Now I’ve tried to turn it back on again several times and I keep getting stuck at the gray apple screen in the beginning. I’m starting to freak out a bit because I need this computer…