Does anyone know what it means if your hot water all of a sudden stops working? But the cold water works just fine?

Do you mean that the water that is supposed to be hot is not hot anymore, or that there is no water coming out of the faucet at all?

If your hot water is running and you don't have hot water then your water heater is the problem. If it's a gas heater it may be the thermocouple that's gone bad, and the pilot light has gone out, if its an electric water heater it could be thermostat that tripped off the power ,or a bad element, call your local licenced plumber to trouble shoot the situation.
If you have hot water every where else except on one faucet and there is just no water coming out at all, then check to make sure that the shut off valve is turned on all the way ,if there is one you'll find it under the cabinet beneath the faucet. Some valves have a rubber seat on the stem that may come loose and cause an ubstruction at the valve. Once again the best thing to do is call your local licenced plumber to take care of the matter, so you don't end up in hot water. Good luck

Yeah... hey! Mommymay... we need some more info. Here.

SP gave some good information... but would be nice to know what you are asking about??

Have you lost hot water at a single faucet or shower valve... or have you lost hot water to entire home?

If whole house issue, let us know how water is heated in your home... water heater (gas, electric, tankless), indirect heating system, etc..?

If single faucet/valve lost hot water, see if you can get us a name on the faucet/valve.

Also, is this something you want to attempt to repair or planning to call in local plumber..? Let us know.

Talk soon... Mark