Your bed is not heating up uniformly or not heating up at all.
Turn OFF the printer FIRST
Check if your bed power switch is on.
Location of the Bed Power Switch
Check the screw terminals that connect the bed to the Duet. Check these terminals on both the Duet side and the heated bed side. It is possible that they loosened up and are not making proper connection.
Feel the wires going into the terminals and move them around to see if they are loose. If they are open the terminal cover and screw them down tight again with a Phillips head screwdriver.