Have run through recommended engine and power start up sequence. Aircraft will take off and fly. It searches out flight plan route and follows it. However, no instruments function, the AP will not operate. Have tried restart in flight, no effect. Anyone please have idea how I proceed? This is my favourite aircraft and it used to work beautifully.

What was this recommended sequence? Did you lose engines besides instrument? Maybe you moved power selector by accident? Or did you turn off the engine gens by turning on the apu gen after starting it? I don't see many options.