We can't get hot water to any faucet, so I checked water heater and opened relief valve briefly. Releif valve released hot water? Why would hot water not get to house?

Blocked heat trap nipple would be my first guess, the older ones with a ball get stuck sometimes (assuming you have them). Cold water throughout the house?

Will draining the tank help? Or is it better to get plumber to change the nipple?

No, draining the water heater will not help if you have a stuck heat trap nipple.
If you have a stuck nipple it will be on the hot water out side if you get the water out the T&P valve.

Something you could try until you can replace the nipple or call the plumber. Might work might not.

Assuming you have clothes washer. Turn off valves to washer. Disconnect one hose at the stop valve, disconnect the other at the washer. Take the hose you disconnected at the washer and connect it to the other stop valve. Open both stop valves. Open the T&P valve on the water heater. Water from the cold side of the washer stop valve will flow to the hot side and back to the water heater through the hot water lines and out the T&P valve. That should free the stuck heat trap nipple.

If this works it will only be temporary.

Thanks for the advice, unfortunately no joy. Plumber will be here in the morning. Appreciate assistance!

Sorry, should have also said close the stop valve on the water heater.

Let us know what the plumber finds.