DIAGNOSIS Grit, scale, or corrosion has gotten into the trigger valve mechanism inside the gun. It's a simple spring-and-plunger design that gets gummed up.
FIX 1) Disconnect the high-pressure hose from the gun. 2) Look for screws holding the two halves of the gun together. Unscrew them. 3) Carefully separate the halves. You'll see the trigger mechanism and a small spring. 4) Clean all parts with vinegar to dissolve mineral deposits, or soap and water for grit. A light coating of silicone grease (not oil) on the moving parts will help. 5) Reassemble and test.
IF THAT DIDN'T WORK The spring may have lost its tension or the rubber seals on the plunger are swollen and binding. Replacement trigger assemblies or entire guns are inexpensive and often easier than a full repair.
HEADS UP When you disconnect the hose, expect a little water to spray out from residual pressure. Point it in a safe direction and squeeze the trigger a few times after turning the machine off to relieve that pressure first.