how can i fix my nissan navara d40 2006 manual 6speed loss of power?

I have a nissan navara d40 2.5L turbo diesel 2006 its only done 21000kms and its just lost power completely. the rpm only reaches around 2300rpm and doesnt go any higher, at idle, and all gears.
Can anyone tell me whats going on with the piece of junk?? I have tried 3 dealerships and no one has any answers. Its a pain having to only drive at 2300rpm max.