Step 1: REMOVE YOUR BATTERY. you don't want to be dinking around with a dodgy throttle with the battery installed.
Step 2: Inspect the throttle closely to see if something is wedged in or binding. Sometimes the grip can shift and rub tight against the throttle.
Step 3: If it's not binding, but it doesn't return to "neutral position" then you may have a broken return spring in which case see Hoggdoc's recommendation and order a replacement.