well first thing that comes to mind is the wiring in the mast and pantograph like stam suggested, this generally is the most common cause of function failure in a reach truck, damaged wiring. If the wiring isn't secured well it can get cut or pinched, if oil leaks onto the connectors it can interfere with connections also.

as for the diode pack? they are generally used to isolate circuits to prevent voltage feedback into other related circuits. Kind of like an electronic flow valve so to speak. It is designed to allow multiple functions to operate with no feedback into other circuits. If you have a bad diode you will either get blown fuses or multiple functions trying to operate at the same time when ONE function is selected and operated.

Since you have NO operation at all for your reach function i would start checking switches, then go to the solenoids that operate the auxiliary functions(generally a manifold block with several solenoids mounted on it somewhere on the carriage or could be under the dash panel). Then check the wiring in the mast. And by all means check all your control fuses too.

question: is it JUST the reach function not working or have you lost ALL your auxiliary functions on the pantograph? This will determine where to look for the problem. If you lost all functions then start at the control handle and go from there, if you've lost only the one function then still start at the control handle but most likely you will find the problem out on the wiring in the mast and pantograph area.

does this truck have wiring going through the mast to the carraige? do you have a sideshifter? where is the "diode pack" located?