Why won't my iBook startup?

Actually, it does start up but fails to go to the login window. Instead, it just stays stuck on the apple logo (with the little spinning thing underneath it) and then after a few minutes goes to a blue screen. Nothing more happens. This started yesterday, and the laptop has not been dropped or mistreated in anyway. It can’t be anything associated with HD corruption as my hard drive shows up when I hold down option at startup, but nothing more happens… Any help greatly appreciated.