I think it is corrupted, as others may have already stated. That's exactly how a reference problem OR corruption shows up - real common, native VBA functions like left, right, instr, will suddenly act as if they are unknown.
Of course not all reference-related "problems" will show up as missing or broken; you can code a reference to an unreferenced library, but then again, that should only cause a compile error - not what you are experiencing.