The Classic Machinery Network

We have a JCB loadall 520/50 which we only use about the yard , but is nevertheless very useful indeed . Just suddenly it was working normally , and then decided to refuse to engage reverse . Where should I be looking as a first attempt , solenoids , bad connection on lever perhaps , or where ? Hope it isn't a proper transmission trouble .
Any ideas gratefully received

check solenoids on the gearbox they break often

also check fuses and relays