I returned from a 2 week trip and found my hot water tap is dry. There has not been any freezing (seldom is here in Southern California!). What are other possibilities as to why the water would have ceased to reach the tap?

Sounds like some trash or debris got into the inlet ports on your cartridge and is now blocking the flow.
Turn the water off and open up the valve. Remove the cartridge and check and clean the inlet ports.
Now place a pan over the open valve and turn the water on to flush out the supplies. Reassemble and test. Good luck, m Tom

What is your piping material? Galvanized pipe could be corroding the pipes closed. Debris in the cartridge is also a possibility.

I agree, most likely junk in the hot inlet. However can you confirm that we are talking about a single hot fixture and not ll/several fixtures?