When you boot up hit ESC and see what message is on the screen. Also, simple enough, try a different keyboard. On systems with PS/2 ports check to make sure the mouse/keyboard is hooked to the correct (green/purple) port. Also try unplugging everything except your keyboard -- I've seen some systems where the keyboard just won't work on certain USB ports.
A system will actually run without a battery, but, you have to reset the time, settings, etc, each boot.