Lack of electrical power, problems with the up/down activation switch or a failed vent gear motor can prevent the downdraft vent from going down when you press the up/down button.
Follow these downdraft vent troubleshooting steps to find and correct the cause of the problem when the vent won't go down:
Turn on the fan to see if the downdraft vent has power. If the fan doesn't run, check the house circuit breaker. If the breaker tripped, reset it and see if the vent will go up. If the vent doesn't go up, check the electrical outlet to make sure it has power. Plug a small lamp or appliance into the outlet. If the outlet isn't working, have an electrician repair the electrical outlet.
If the fan runs, then you may need to adjust the position of the up/down activation switch. To check the up/down activation switch position, unplug the downdraft vent and remove the switch cover from the right end of the vent. Check the position of the up/down activation switch plunger. If the plunger isn't touching the bottom of the vent's button membrane, loosen the up/down activation switch mounting screws and move the switch up until the vent's button membrane barely touches the plunger. Replace the switch cover and restore electrical power to the downdraft vent.
If the up/down activation switch position is okay, disconnect power to the downdraft blower and check for continuity through the up/down activation switch using a multimeter. If you measure no continuity through the up/down activation switch when you press the button to lower the vent, replace the up/down activation switch.
If the up/down activation switch works and the vent gear motor doesn't run when activated, replace the vent gear motor.
These basic downdraft vent troubleshooting steps work for most Kenmore, Broan, KitchenAid, Frigidaire, Electrolux, GE and Whirlpool vent systems.