Assuming that the light is fed from the outlet box, it could have been wired at least three ways (and more, actually):

The light could be fed from the load side of the GFCI receptacle (in which case there should be 4 wires connected to the receptacle--a white and a black each at either end). The light could draw power from the line side of the receptacle (in which case there should be four wires connected to one end of the receptacle. Thirdly, the light wiring could bypass the receptacle by pigtailing directly to the incoming feed cable.

Any of these can be valid ways of wiring a light from a bathroom outlet. The question is: how does yours appear to have been wired?

I'll admit to being confused by your terminology. When say "outlet box", what does that mean? Do you mean the receptacle? If no white wire was attached to the receptacle, I don't see how it could have worked under any circumstances.

It strikes me that it is much more likely that the problem lies with the wiring (probably one or more loose or disconnected white neutrals) than with the GFCI receptacle itself. However, we'll need more precise information to diagnose.

Can you detail all of the wiring you see in the box or post a picture?