1) First thing to do, no matter the issue, is to check the transmission fluid
The transmission is slipping: odds are the fluid level is low and topping the fluid level off will take care of the immediate issue(you'll still want to find out what happened to the missing fluid.
The transmission fluid is Dark and smells burnt: At this point if the transmission is shifting ok, a fluid and filter change is in order. If the transmission is causing you an issue and the level is full but still burnt looking you are looking at needing a rebuild. The dark color comes from the worn clutches inside the transmission. The clutch material is similar to that of brake pads and when they wear the leave dust similar to brake dust collecting on your wheels.
Transmission won't shift, No reverse, Stuck in Gear, No Overdrive:
these transmission problems can be caused by severe internal damage, a failed shift solenoid, failed sensor or a transmission computer module failure. Sometimes a simple Read more on Transmission Troubleshooting