Opening a valve does not allow water to flow

We need more details. The word valve is not usually used when talking about plumbing. Are you meaning faucet or spigot? Where, hot or cold or both?

This is a valve on the output pipe of the hot water heater that feed the faucet in a downstairs bathroom. When open the valve the water does not flow. Maybe I have to replace the 'guts' of the valve? Thx, Tom

No water out of that faucet at all ? Try to remove aerator on that faucet to see if that restore water flow...

What kind of valve is it ? Perhaps, you could snap and post a picture of it ? Back to you...

So is it just the hot side of this fixture only, you get hot water everywhere else in the house, right? Can I assume you have just this one water heater for the entire home?