You need to check the 12/4 cable, going to the platform, for power going to the platform. In the lower control box locate the Black, Orange, Red and Blue 12 gauge wires at terminals 31, 32, 33 and 34. The Black wire on terminal 31 is ground for the platform. The Red wire on terminal 32 is 12 volts to the platform for the horn. That should have 12 volts when the master switch is on. The Blue wire on terminal 33 is ignition power from the platform. That should have 12 volts on it with both E-Stop buttons (base and platform) are pulled on and the select switch in platform position. The Orange wire on terminal 34 is 12 volts to supply all the function controls in the platform.
Possible causes why the engine will not stay running (I'm assuming the engine does not crank either) are a bad contact block on the platform E-Stop button. A bad select switch in the ground control box. A bad circuit breaker in the platform control box or broken wires between the platform and base. It has been a while since I've worked on an HA so I'm sure I'm missing something. So while I'm trying to remember check for voltages at the 12 gauge wires described above.