Take the fog machine to a well-ventilated area. Drain all fluid from the reservoir.
Fill the reservoir halfway with the distilled water, then add distilled vinegar until you fill the reservoir. The vinegar will dissolve whatever is clogging the machine, and the water will flush it out.
Plug in the fog machine, let the heating element warm up and turn on the fog machine. Let the the machine run until the normal fog flow is restored.
Unplug the machine, and drain the excess water-vinegar mixture from the reservoir. Fill the reservoir with the manufacturer's recommended fog fluid.
Plug in the fog machine, and let the heating element warm up. Turn the machine on, and let it run until it produces fog.