Hi
Remove the cover off of the engine and locate the gear change and throttle controls on the end of the two cables.
the easiest check is to move the throttle lever and see what happens, the gear change should move first and when fully engaged the throttle control should start to move immediately after.
a better check is to disconnect the cable linkages at the engine end and see if you have excess play in the throttle cable [which sounds like the case] if so you need a new cable the same length and type.
Probably a "morse" cable, you may need to remove the old one to get exact length [if you do put a cord through as you extract the old one, so it can be used to pull the new one through], the type is usually shown by the sleeve colour
Probably nothing to do with hitting the rock, but could have been done I suppose when investigating what had happened
Hope that helps
Charlie