While driving at about 20 mph pull shifter into low gear to see if you have manual low gear. If not, low + reverse clutches are shot and will require overhaul. If you have manual low possible shift linkage-valve body problem. Need to see the trans. shop! By man. low i mean does it slow the vehicle down or just freewheel when pulled into low, not starting out with shifter in low.
I assume you have a automatic transmission? Check your fluid level and check shift linkage to make sure there is not so much wear in linkage that it could not be moving shift linkage in transmission full range. Other than that the problem would be internal in the transmission and would most likely mean a compleat overhaul or a replacement unit.