Most likely the cable from that lever slipped in the holding clamp (by carburetor). Loosen that clamp and see if it speeds up when you push the throttle with your finger or a tool.
If that dont do it, the power needle on carb needs adjustment, or the governor is out of adjustment. If you adjust these things, be sure to mark where they were (how many turns you made). so you can set it back if thats not the problem.
One other thing, could you have some bad or old gas? A little carb cleaner in the gas tank never hurts either.