If your battery gets low when using an application that fills the entire display on a portable Mac (that is, the Mac OS X menu bar is not visible), you will not see the low battery alert warning. This article applies to Mac OS X 10.2 or later on an iBook, PowerBook, or MacBook Pro computer.
When this happens, your built-in keyboard and trackpad may become unresponsive.
Simply connect your computer to an AC outlet to recharge the battery. If your AC adapter is not handy, connect an external keyboard and mouse to your computer to see and dismiss the low battery alert (after which your built-in keyboard will work).