HP Deskjet keeps saying it's out of paper but there's paper in the tray?

[Ich hab' Dich lieb]

It won't pick up the paper, it makes noise like it's going to but it doesn't. I turned it off and on several times and tried printing again but it won't print. I've used this same printer on this same computer many times before without this problem. I took some of the paper out, put more paper in, everything I could think of and it won't pick up the paper. The model number is f4580.
I'm using a Mac with Snow Leopard if that matters.