The cold water supply valve to the water heater is open, but no water is flowing. I opened the drain valve on the heater and no water came out. Obviously it hasn't been filling for the past few days. I have plenty of cold water at all my faucets. I have shut off the breaker.

If it is a gate valve, the disc could have separated and even in the open position will not allow water to flow. Turn off the house water, release the pressure and try to take the valve apart. See if the valve is actually open.

So the valve immediately located at my water heater could have failed, even though it has been open for years and untouched? Do I just need to replace the insert part and are these parts standard?

You mentioned no water out of the drain. When did you flush the heater last? To plug a 3/4" fill line takes a lot of debris. Are you on a well? Rust in the water? Copper or galvanized fitting after the valve? Have a filter on the tank line? For the disc to come off without opening or closing would be a little strange. I would close the valve and try to open it again. One indication of a disc off is that you won't be able to shut the water down since the thread on the handle hits the disc. You will need to do a little more snooping, particularly since you have CW. Any chance the CW feed line to the heater is frozen?

Flush not prob. Copper lines & fittings o.k. I have city water. The heater is at the end of water line inside house 7" from exterior wall with an outdoor faucet at the very end. Frozen? Possible... was cold as hell yesterday & today in N.Texas

Turns out line was frozen past the gate valve so water pressure could not force hot water out. I opened the closet door where water heater is located, allowing the heat from home to penetrate walls & defrost pipes. Hot water restored!