If your home is without power for an extended period of time, unplug appliances and turn off circuit breakers. This will prevent surges when the electricity returns.
Once you have power back, make sure there is no standing water in your basement. If your system requires service to get it started again, for safety reasons it cannot be worked on if water is pooling around it.
If floodwater reached your heating system, get an inspection done before trying to restart. The valves and controls are vulnerable to water damage — even if it cannot be seen. Corrosion begins inside the valves, and damage may not be apparent, even if the outside is clean and dry.
If the area around your heating system has remained dry, check that the system’s power switch and circuit breakers are back on before you try restarting.