I have a Hayter 13/30. The gears have stopped working?

Hi, I have a Hayter 13/30 garden tractor. The gears have stopped working, there is no tension on the gear stick at all. All I can find online is the basic manual, would like to find a servicing manual so that I can at least try and fix this thing. At the mo, I can't even get any panels off so that I can look at the innards! Help please!
Thanks