I assume the same truck and the same trailer were connected and the brakes worked before now. The potential failure points (in no particular order) are the electric brake controller, the wiring to the truck trailer outlet, insufficient trailer ground, trailer wiring, or the trailer electric brakes. Here are the steps I would take to eliminate them one-by-one. Some of these you have already done. 1) check the trailer outlet on the truck (specifically the brake prong) for voltage (12v or a bit higher). Have someone apply and release the brakes in the truck while you are testing. The voltage should vary. If no voltage, it's either the wiring to the trailer outlet on the truck or if the voltage is constant, the truck's electric brake controller is defective. 2) If the truck's trailer outlet has voltage, you've eliminated any issue with the truck. So the issue must lie with the trailer. Connect the trailer plug to the truck outlet. Check the ground wire on the trailer to ensure the ground wire has no corrosion and is firmly attached to the trailer. 3) If the ground is OK, then check the brake wire on the trailer for voltage while the assistant depresses and releases the brake pedal. 3) If the trailer brake wire has voltage, you've eliminated the trailer wiring. The issue must then be with the electric brakes.