The simplest problem could be loose wiring within the OBD II connector or blown DLC (data link connector) fuse. The most complicated fault could be a problem with the Engine Control Computer (ECU).
If you can purchase an inexpensive trouble code reader/scanner (Pepboys or Autozone carry them) go ahead and try connecting to your vehicle's ECU via the OBD II connector at home. If the scanner lights up as it's connected to the DLC this indicates there is power at the connector and the problem is more then likey with the signal wire between the OBD II connector and the ECU. If the scanner does not light up this might indicate either a loose or disconnected power or ground wire at the connector, and/or a shorted fuse. Check your vehicle's fuse box for a fuse labeled DLC (Data Link Connector). Ensure it is not shorted.
