If the water has stopped coming out of the hot tap (not on sealed systems or primatic cylinders - see bottom of page for these) and you've already checked it's not turned off anywhere, then chances are it's airlocked.
ALLOW HOT WATER TO COOL BEFORE ATTEMPTING EITHER TECHNIQUE.
Well what to do?
Connect a suitably sized hose pipe (dependant on size of tap) between the affected tap and a working cold tap (usually next to it).
Secure with jubilee clips at both ends.
Open the air locked tap (hot tap) fully, then slowly open the cold tap.
Allow water to run for a few minutes from the cold (working) tap to the airlocked hot tap. This will push the air lock out.
Then turn off cold tap first then the hot tap.
Remove the hose and check operation of all hot taps in the home. If not worked repeat until clear.
If you have a mixer tap this is even easier, but possibly messy. Don't attempt if you think you can't hold the tap firmly for awhile. Or if you think you'll mess it up.
Hold your hand firmly over the tap spout (where the water comes out)so it is sealed.
Open hot tap and then slowly open the cold tap allowing the water to push out the air lock.
Allow to run for a few minutes.
Close cold tap then hot tap.
Check operation and repeat if necessary.
Other possibly causes
Check all the valves are open on the hot water pipework. Maybe one of the valves has snapped shut or half open (when someone has closed it). Will require draining down and further investigation.
Check that the pipework on the hot water isn't dipping the wrong way causing it to periodically air lock. It should be rising slightly from the top of the cylinder to the tee (where the open vent meets the hot water draw off).
Blocked pipework - rare but does happen.
Faulty tap, will require further examination. Possibly repairable if not then replace tap.
Sealed System Or Primatic Cylinder
Sealed systems don't really get airlocked as the hot water is under mains pressure. So it is either one of the following - a valve somewhere on the pipework is closed, blocked pipework, a faulty tap or the supply of water has been turned off.
You can't remove an airlock on a primatic cylinder with a hose pipe at the taps. The reason being is that this can cause the hot water and the heating water to mix. You will have drain down the hot water and fill it up again. Also check the pipework has the correct rise to where it vents the air.