There is no way of telling from here. It will be a double breaker, most boxes will have 3 doubles. One for the stove, one for the AC and one for the water heater. Process of elimination should tell you which is for the heater.
Assuming you are referring to a 240 volt residential hot water heater you will need to install 10/2 NM cable (Romex) with ground from the service panel to the water heater wired to a 30 amp circuit breaker in the service panel. If you cannot see the water heater from the service panel you will need to install a service disconnect at the water heater.
10/3 wire with 30 amp double pole breaker from panel box to water heater.
You can pull the front panel off the Hot Water tank and there is a red reset button on the electrical that controls the power to the elements. Or, try resetting the breaker on the electrical panel serving the tank.
Yes you need to shut off the Main water valve. Usually found in the basement or outside the house near the meter. Also you need to shut off the Gas valve located at the water heater, or the Breaker located in the breaker box/ fuse panel. Sometimes the water heater will have its own Switch box, located on the power wire
Because hot water heater breaker has tripped, or hot water heater has failed.
turn off your water
The wire size is dependent upon the wattage size of your water heater. The higher the wattage the larger the wire size. A standard 3000 watt heater will use a two pole 20 amp breaker connected to 2C #12 wire.
Yes
Sounds like it is a 220-240 Volt hot water heater. The black and red are connected to the 220 volts supply and the white is connected to Neutral. At the breaker panel red and black connect to the 2-pole 220 volt breaker and white goes to the neutral bus bar.
There is a thermostat inside an accessible panel on the hot water heater. The panel is typically secured by two screws. When the temperature goes below the set level the heater turns on. You'll either see a specific temperature or just an adjustment that says hotter. Some installations have an external timer that might turn off the hot water heater between midnight and 5AM, for example.
The wattage of the water tank is needed to size the breaker and the wire to feed the tank.
YES!!