Theres 2 possibilities: Electrical failure or mechanical failure
If you hear the solenoid buzz when you try to open the coupler and it doesnt open, then try pulling on the knuckle as you press the button. If it opens, then shoot a puff of dry graphite into the knuckle and continue open and closing the coupler until it frees up.
If you dont hear the solenoid buzzing, then chances are you have something electrical going on. Either its unplugged or a broken wire somewhere between the board and the coupler. Flip it over and check the solder points on the coupler, thats a common failure point for the wire to break.