Watch the dome light when you try to crank it. Does the dome light go out or dim (should dim a little bit, but not much)? If so, either something has drained the battery and you need a jump or you need to recheck your battery terminals because one of them is not making a good connection... try to twist the terminals on the post; you shouldn't be able to move them. If you can, they're too loose on the post. If not, take them off and look for corrosion or see if they need to be cleaned. Posts may need to be cleaned too.