What year is it? If pre-2018, it was on the rear drive line with a rotor and two pads that were cam-actuated with a cable that is adjustable. People always adjust the cable... but forget the cam plate. If it rotates far enough the goes past the cam's ramps and sticks, its there until someone helps it back. If this is the case, reach in and with the brake lever off, force the arm back down. That will force it back on the ramp. To adjust it, turn the cable adjustment most of the way back in, then a turn or two back toward tighten. Then locate the bolt and it's lock nut in the center of the cam. Loosen the lock nut and turn the bolt in until there is only about an 1/8" of movement in the arm. It won't take much. Then tighten the lock nut. Then adjust the cable to your liking making sure the pads are NOT baring any pressure against the rotor when off. A little loose is better then too tight.
If this is not the issue, maybe the cable is rusted-up and needs replaced.
Here's what it looks like:
