Help trailer brakes went out (UPDATE)

Since you said it was intermittent and then went to total failure that does narrow it down. A missing ground is one possibility but a very common issue, especially with intermittent brakes, is how the wires run to the wheels. They usually run down one side or the other and then go through the axle to get to the other side. Unfortunately the inside of most axles is not smooth and the OEMs do not sheath the wires. Normal vibration then rubs bare spots in the insulation causing intermittent shorts and eventually failure. While this might not be your problem it is a very common cause of brake issues.