Make sure all the valves are fully open.

Scale can break loose in a water heater and partially obstruct any valves in the line, but the small stop valves under the sink are especially prone.

That is one of the reasons I prefer ball valves for stops that are not visible.

Is there a mixing valve in the single handle faucet that could be clogged or bad?

Problem solved! There was a big chunk of debris lodged in the hot water side of the inner part of the faucet.