The hot water stopped running through just one bathroom faucet. (It's a two-handle system). I turned off the hot water supply line valve under the sink and removed the valve inside the handle. But even when I turned the supply line back on, no water came out of the handle. There's a small, short-lived drip from the supply line valve when I open it, but it stops after I shut it. I've never seen anything like this. Any ideas?
The solution turned out to be disconnecting and reconnecting the inlet side of the supply line, because -- here's the kicker -- the supply line is a Floodsafe line designed to automatically shut off completely if the flow rate exceeds a preset maximum, which I guess I exceeded momentarily. I didn't see the labeling on the supply line until after I disconnected it, and this is the first time it happened in 1.5 years of living here. Frankly, I don't see the advantage of this. It won't do anything to stop common slow, drippy leaks.
The faucet is plugged somewhere before the faucet valve (that you removed). Disconnecting the supply line (described below) should confirm.
The connection from the shutoff valve under the sink to the faucet (the supply line) is plugged. They're often plastic or steel braided lines and are replaceable, try removing it and see if it's clogged or the water now runs through (either out of the shutoff valve directly, or the end of the connection line). Like the braided stainless steel lines in this image, that curve around from the shutoff valves to the copper faucet attachments:
The shutoff valve itself is plugged. If it's the type with a removable washer, try taking it apart and see if it's ok, but TURN OFF THE HOT WATER FIRST! Once you take apart the valve water will spray out uncontrollably. You probably need a friend at the main hot water shutoff valve to briefly turn on the water to see if it comes out.
Like this type (from Home Depot) where you unscrew the part just under the handle then the "innards" unscrew:
Apparently it's called the "packing nut" you want to unscrew, then the valve stem & everything unscrews, like in these images: (But don't remove the shutoff valve unless you absolutely have to, the washers/packing is easy to replace, even the whole valve stem & "innards" usually match new parts.)
If it's a quarter-turn ball valve they don't generally come apart, but you might be able to try putting a pipe cleaner or tooth pick (something non-scratching) through the open valve to see if it's unblocked.
The hot water line running to the bathroom is plugged somewhere. Does the shower still have hot water? They usually share the same line closely. If the shower's hot then it could be some damaged pipe/line inside the walls somewhere, and a plumber is in your future.
All the hot water in the house is off. Turn it on.