Could be a few things.
Worn or frayed throttle cable, especially if the throttle grip is returning to closed but the bike keeps reving. Cables will wear out over time. If it is frayed underneath the shield, you won't be able to see the problem. Cables are usually pretty cheap (under $20).
If the throttle grip is not returning to closed, could be some grit between the tube and the handlebar, or the grip and throttle tube got loose or pushed in during a crach.