Reset the RCD - something made it trip. If it refuses to reset begin unplugging things until you find the cause - or better still unplug everything and begin plugging things back in one at a time.
RCD is an excellent safety aid that protects against electrocution. Usually the whole house Lights and Power are wired through the RCD (a bigger switch) so you should be in the dark.
If it's a small breaker that's tripped it's an MCB and that will have tripped because of overcurrent in the power circuit - unplug things and reset it. If your lights are still on it's probably not the RCD tripping. Try not to let your fridge defrost.
Peccavi, January 2009