Diagnosis
The trigger mechanism is either jammed with dirt/debris or, more commonly, the internal safety lock is engaged. There's a sliding lock on top of the gun to prevent accidental triggering.
How to Fix It
First, look at the top of the gun, right near where your index finger would be. You'll see a black plastic slider. If it's pushed forward toward the nozzle, it's in the 'locked' position. Slide it back toward the handle. You should hear/feel a click, and the trigger will now pull. If it was already unlocked, try pulling the trigger while pressing the slider back and forth—sometimes they get out of sync.
If That Didn't Work
If the slider moves but the trigger is still solid, something inside the gun is jammed. Grit can get into the mechanism. You can try spraying a little silicone lubricant (not WD-40) into the trigger pivot area while working it back and forth. If that fails, the gun may need to be replaced.
Heads Up
Before connecting or disconnecting the high-pressure hose, always engage the safety lock on the gun and pull the trigger to release any trapped pressure in the hose. This prevents dangerous 'whipping' of the hose.