In the event of a power failure first check you consumer unit (Fuse Board) to see if the RCD has triped.
If it has it will look like the one below.
Tripped Hager RCD (other manufacturers may look different to this
To reset an RCD move the grey toggle switch to the other position; if down move up, if up move it to the down position. If the RCD stays reset, the main toggle switch does not go back to the other position, the problem is solved.