How can the landing gear of an A320 be lowered in the event of a simultaneous hydraulic and electrical failure?
The A320 has an emergency gear extension handle to deploy the landing gear by gravity in case of electrical, hydraulic, or mechanical failure. To force the landing gear to extend, one of the crew must pull the emergency gear extension handle up, then turn it clockwise three turns. The cutoff valve then shuts down hydraulics to the landing gear system and depressurizes it. This allows the landing gear to fall free to the down lock position by gravity.

Source: a320dp.com