I agree with Ian – it is most likely something in the pedestal that is fouling the movement of the lever in the Ahead position. It is very unlikely to be anything in the engine itself.
The only other possibility I can think of is that the cable to the gearbox may have slipped in its termination, either at the gearbox end or the throttle lever end.
It should be easy to check the engine/gearbox end of the cables, especially if you can get someone else to move the throttle forwards and backwards while you observe the cable movement on the engine and gearbox.
If the engine end looks OK, then you will need to gain access to the back of the throttle lever inside the pedestal and again, watch the cable movement as you move the lever.
Hopefully the reason for the problem will be clear once you can see the mechanism.