Looking for some advice.

WE have a crown ride on pallet ride jack. Its an older unit but honestly a life saver.

the machine goes up and down.

the machine goes forward when pressing the red emergecy swtich.

The machine does not work forward or backwards when we pull the handle down and use the twist grips. but the horn, etc etc everything else works.

Where should is start looking ? any specific part that needs to be replaced? fuse? what part of the machine should i start opening?

Any help is greatly appreciated.