I had the same issue once. The connection at the transmission where the gear shift lever cable connects is worn out (plastic parts). When the plastic part wears out, it allows the gear selector cable to come out off the lever on the transmission. this requires a new plastic "bushing" to be installed. You can visually check it, it is located on the transmission near the firewall, you will see a lever about 2-3 inches in length where the cable connects to.
