I have a 2000 F250 super duty diesel. I pull a living quarters horse trailer. When I plug in the trailer 7 pin plug to the truck a fuse on the trailer battry blows, and the trailer brakes freze up.

 

Might want to post this in the Towing and Hauling section of the forum.

 

Your trailer battery is probably discharged, making it draw too much from the truck, blowing the fuse. When the fuse blows, the breakaway box thinks the trailer has been disconnected from the truck & locks up the brakes.

Charge the trailer battery before any further diagnosis.