I had a similar problem but mine was the positive lead coming off.
As the bed bled the heat from my soldering iron I couldnt get the pad hot enough to solder it back on.
Contact support by email and they will send a new bed out. To speed things up take a photo of your serial number on the bottom of your power supply as they will ask for that. Yeh I have been there…
The reason mine failed was that the main part of the lead wasnt zip tied and the back and forth motion caused the cable to get brittle and snap 