Okay, who's been messing with your system? Ask around, if someone wails "I didn't do anything!" wait for them to add "except...":-).
Whatever they did, they reinstalled or rebuilt PHP without GD_lib support. What needs to be done depends on what OS you've got; for windows you need to make sure you've still got a copy of the gd_lib DLL floating 'round, and make sure php.ini has support for it turned on. On Un*x you need to make sure you've still got gd_lib installed and rebuild PHP with --with-gd (as well was whatever other configure options you have; call phpinfo() and see).
Better yet, if someone else is responsible for the installation in the first place, delegate the task to them (they're probably the ones who broke it in the first place!)
