Transmission

The prndl switch basically just tells you what gear the transmission is in. P-park, R-reverse, N-neutral, ect. This won't cause you to have no reverse unless it is indicating it is in reverse and you actually have it in a different gear. If you don't have any reverse no matter where the shift lever is, there is probably an internal transmission issue. If you have a way to read the clutch volumes with a scanner, you should be able to see if there is an unusually high value for the reverse clutch pack indicating a leak in the seal that holds the fluid that applies the clutch pack. The normal clutch volumes are listed in a service manual if you have one. The other way to check for internal problems is to remove the pan and look in it for clutch debris which looks sort of like mud. This is caused from the clutches slipping and burning the material off of the clutch plates and landing in the pan. Either way you will most likely need to rebuild the transmission.