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There are quite a few things that can cause this.
When the transmission control module sees a problem it thinks can cause potential damage to the trans it can lock you in a single higher gear. When this happens it sets fault codes which are needed as a starting point for diagnosis.
The problem could be internal in the transmission, one of the shift or pressure control solenoids, the range sensor, temperature sensor, speed sensors, wiring between components or the TCM itself.
If all fuses are ok then the next step will be to see what fault codes are stored in the TCM. This will need to be done with a scan tool or code reader, most larger parts stores like Autozone will read fault codes for free in the parking lot. Once you know what codes are stored then you can diagnose it from there.
I never thought to check the darn fluid and heard that can also be it! Dont know if the engine is supposed to be running or not when checking the Tranny Fluid?