we would need a bit more information to hazard a guess. Explain the what exactly yu replaced and what the system consists of.
- Are the brakes auto adjusting or manually adjusted?
- When the drivers side wheel was locked up how did you get it unlocked?
- If you disconnect the battery and unplug the umbilical do all the wheels roll freely?
My guesses are
You pinched one or many wires when you replaced the backing plates. Test by disconnecting the umbilical and disconnecting the battery on the trailer.
If disconnecting all power does not free the locked up wheel that brake is adjusted far too tight.
If they are auto adjusting brakes. Make sure the backing plates are on the correct side of the vehicle. If on the wrong side then when in motion the brakes will adjust tighter with every application of the brakes.
It's possible your tow vehicle has a blown towing fuse or problem with the trailer connection.
It's possible the trailer umbilical is damaged.