I am a cleaner. I apparently got water into an external powerpoint
If you got water into an external power point, it might trip an RCCB (gfci, ground fault circuit interrupter in the US). I would say to look first for external power points near where you washed. Remove the cover plate and  blast the insides with a hair dryer, being careful not to overheat the power point. If the owner has no power at all, they will have to use an extension cord from a power point of the neighbor's house.

I had a roof leak cause a loss of power on one circuit by dampening an external power point on a circuit with a GFCI breaker. Until I could get the roof fixed,                    I removed the receptacle and wired the circuit straight through with twist on wire connectors pointed upward so they shed water. 

If an external power point was not properly sealed, it is the home owner's fault not yours. But in future you should not use a strong stream of water to clean around windows. I think people have gone overboard about 'power washing' houses.

If there are no external power points near where you washed, look for power points inside near the windows and dry them with the hair dryer.