Electric motor or wiring failure. Debris in gun valve assembly. Clean valve assembly or replace gun. Water turned off. Turn water on.
Nozzle is plugged.
Clean or replace with proper size. Inlet chemical injection valve is open, without the end of the pickup tube inserted into detergent. Close soap valve or submerge detergent pickup tube into solution. Coil on hot water machines is obstructed. Clean obstruction or scale deposits from coil with coil cleaner.