as was suggested by the other poster, yes, look at the BDI LCD screen and when you try and drive it the LCD will display a code if there is an active fault when trying to drive it.
and yes, if you pull the tractor motor cover and look on the curtis controller there should be an amber light that flashes a steady slow flash if all is ok, if there are any codes present it will flash more quickly the code numbers in succession, then pause and repeat the code(s).
You may also want to check the steering board, it will have several red LED's across the top, they will be lit up according to the mode the system is in, if there are any errors they will be lit up according to what the error is.
These units have had a history of steering faults that will disable the drive.
One of the most common problems is the helical coil connector on the steer direction potentiometer breaks, when that happens it disables the drive. To look at it is hard to tell if it is broken, generally it has to be removed to tell.

Just make sure the battery is charged up, low battery voltage can cause issues too.
As far as accessing the service menu you can do that but it's not going to tell you too much about what's wrong, it only helps when your changing a directional pot or making adjustments to the system to make it run differently and enable/disable options or check the BDI calibration, or check the max height in inches, stuff like that.
Just look on the LCD display and see what codes it shows or what the fault is when you try and drive it, and/or look on the motor controller and steer controller and see what the LCD lights are doing and get back to us.

Have not worked on this truck in a while but l think that there should be a code in the bdi screen also if you pull the cover on the top the plastic cover and open the doors the traction controller Curtis and fuses are there check them and see if the small light on the box is lit it could code there also l believe you can look at most things if you turn the key on and hit the horn button two times very quickly then use the forward and reverse handle to look at the throttle and other inputs if you get the code get back to me l will try to help you the traction controller is black and l believe the steering controller is silver.hope l was some help.