If you have your own water heater there should be a shut off at the heater. If not I would look under the kitchen sink. There may be shut off valves at each fixture. If there is a central water heater in the basement, there would be a shut off to each unit.
I do not live in a condo thus I have no need to shut the main valve
the hot water valve shut off valve shuts off the hot water. The cold water valve shuts off the cold water. ;()
If the shut off valve turns but does not shut the water off, the washer in it has split and probably made it to the faucet. It was blocking part of the hole on the hot side and at some point either shifted or made it through the faucet and in in the screen in the end of the spout. Shut the main water off and replace the washer in the shut off valve.
Showers and tubs do not usually have shut off valves on the supply lines. To shut the water off you will have to shut off the main water line. There should be one on the cold water line to the water heater. This may shut both the hot and cold off or just the hot. There will be one in the main line coming into the house on the basement wall closest to the street or at the water meter.
Which water you talking about? Under the kitchen sink there should be a C/W shut off If your talking toilet there should be a shut off left hand side under the tank. Shower or bath tub possibly integral stops Main cold water shut off Normally by the house trap. Multi story building a cold water riser
Someone shut the hot water supply off to your unit
You will get some back pressure from the water heater. It will come out of either the hot or cold. Shut the tank off if there is a shut off there.
By hiring a plumber...
If electric shut off the current to the elements, if gas shut off the gas valve. Open a hot water faucet to give it air, open the drain valve on the bottom of the heater.
If you want to shut it down completely, turn off the cold water stop going into the hot water tank. If it is an electrical tank shut the breaker off to the HWT. If it is a gas HWT, shut off the gas valve as well.
Backflow from the sewer or drain pipes.
Check the shut-off valve above it. It may be turned off or plugged up.