This is a purely uneducated guess. I suspect the problem will lie in one of two places.
Either it's in the gearbox itself, or quite possibly it the control cables to the box.
If it's the first, that's a specialist job. If it's the second, then it may well be worth going over the connectios at the lever end for anything having become disconnected under the cover, or at the gearbox end, where the cables connect to the operating levers. It looks as tho' the 2 cables "snap fit" onto the levers, and there's also a kick-down control as well.
I suggest a close examination of those points would be about the only practical thing you could do for yourself.