You've likely engaged the safety lock, which is a good feature to prevent accidental spraying. Here's how to manage it.
To engage the lock (like when you're taking a break): Push up on the safety lock on the trigger handle until it clicks into the slot. The spray gun cannot be operated when locked.
To disengage the lock and use the washer: Simply pull down on the safety lock to release it from the slot. You should then be able to pull the trigger normally.
If that didn't work: If the trigger mechanism itself feels broken or the lock won't budge, there could be internal damage. Do not force it. For a broken trigger mechanism or if the gun is leaking, you'll need to call Snow Joe + Sun Joe customer service at 1-866-SNOWJOE for assistance.
Heads up: Always engage the safety lock, turn the switch to OFF, and unplug the machine if you're taking a break of five minutes or more. It's a key safety step.