Answer
Depends if you have an automatic or a manual transmission.
For manual: it could be a misadjusted or broken clutch cable, preventing the clutch from disengaging.
For automatic: under the car, look for a missing or broken vacuum hose. It's supposed to connect to the transmission about 2/3 the way back on the right side. This hose may have fallen off, have a hole in it, or be disconnected from the vacuum source in the engine.
Another cause on automatics is sticking valves in the transmission itself. These can be cleaned and resharpened without removing the transmission.
First answer by Ahc. Last edit by Ahc. Contributor trust: 33 [recommend contributor]. Question popularity: 63 [recommend question]
