Well, not really. A lavatory is closed by maintenance if it's not working. If it breaks inflight, then the FA will call the captain to tell him what's wrong. It is the Captain's decision on whether to close the lav or not, not the FA's.

If the lav was inop prior to departure, that was a maintenance call, thus the reason I think it was broken versus no running water, lavs go out all the time with no running water but with hand wipes.