DIAGNOSIS The 'Keep Hot' function is either turned off, or the warming plate's thermostat has failed. It's a simple heating element with a control switch.
FIX First, confirm the function is on. The KEEP HOT button should be lit. If it's off, press it. The plate should get warm to the touch within 5 minutes. If the light is on but the plate is cold, unplug the machine and check that the carafe is seated correctly, triggering the safety switch on the plate.
IF THAT DIDN'T WORK The thermal fuse linked to the warming plate circuit may have blown. This requires opening the base (see the 'won't turn on' fix above) to inspect. A failed thermostat or fuse means the plate needs replacement.
HEADS UP The warming plate is designed to maintain temperature, not re-heat coffee that's already gone cold. For best results, start with a pre-heated carafe (rinse with hot water) and use the plate immediately after brewing.