Check the level and condition of your transmission fluid. Most of the time
this is done with the car warmed up and idling in park. If it is low then add fluid
to reach the full mark. The type of fluid will probably be specified on the
dipstick. Drive the car again to see if this fixes the problem. If it does
then have the transmission checked for leaks and repaired.
Try stopping and turning the key off for a few seconds then restart and try driving
again. If this fixes the problem, even temporarily, you may only have an
electrical problem. The car's computer can be checked for trouble codes which may
point to the problem.
If you are planning to call for help then have the following information available
to speed diagnosis: