1995 Mercury Sable GS
3 liter
The check engine light is on.
More importantly, when I put the car into drive or overdrive, the transmission stays in 3rd gear, and will not shift up or down. When I put the car into first or reverse, they work normally.
Any ideas?

That is often the result of the drive train control module defaulting to a limp mode when an input is out of range or non existent. Does the speedometer work? Is the fluid level correct? Does the car have OBD II?

This explains how to pull the codes from your OBDI Ford.

On the list of codes the transmission is involved.