I have a 2015 F350 Lariat. The passenger seat is fully reclined and will not go back up or forward. All the other switches are working on the seat. Figured I would ask before I start tearing into it.

 

First check all the fuses. Next remove just that particular switch or get to that motor and remove the connector, whichever is easier to get to.
Like working on power window, say 2 wires on the connector, red and black, when you feed 12v to red and ground to black, say it goes upward, when you flip the polarity then it goes downward. That said you can bypass the switch and manually feed power and supply the ground, just do this for a split second and see if the seat is responding when you are guessing the wire combination. Like power window when it's already all the way UP you can't go up any further, your seat might be in the same situation, so you just have to flip the polarity when it's already all the way reclined. Basically you are trying to figure out the wire combination. Once the seatback starts moving a little, you can stop it and reconnect the connector back to the motor. Something like the regulator might be stuck at full recline position.
When bypassing the switch on power window, never ever press the button or you can have cross circuit and see sparks, blow the fuse or short something, same can be said when working on your seat manually.
While working on it you might want to spray white lithium grease to lubricate just the metal to metal moving parts.