If I'm remembering correctly, a number of EMB-145 operators deactivated the lav potable water due to issues mantaining the storage tank (behind the mirror above the sink). Overall, it may have been due to the new tank sanitization/potable water testing rules that were implimented in the last few years.

Generally, if there is no water for the lav, it's due to some sort of other issue with either the faucet, sink, or the water storage system (and ends up having to be drained out until fixed), unless someone forgot to check the level and request service.