Anyone know anything about woodfinishing?

[Critters2]

My half-bath has a hardwood floor. Hasn't been terribly well kept up, so it's not one of those take-your-breath-away beautiful wood floors. But it is finished hard wood. Well, over the weekend, I picked up a kleenex box that had fallen on the floor, and found it was a little wet, (I think Shamu may have peed on it when he wasn't feeling well). And the floor beneath it is discolored--dark! Almost looks like a burn, but it's a stain, a discoloration of the wood finish.

Is there anything I can do to lighten it again? If not, I'll have to either throw myself on the mercy of my Trustees, or pay someone to refinish it myself. I'm leaning to the latter option, since one of my critters caused the problem.

Any suggestions?