Most electronic components are not damaged by 'normal' cold temperatures. -20C is not in itself a problem.
Condensation can be a problem, if the temperature cycles between warm and cold with humidity.
Batteries lose capacity in cold; so some battery powered devices might not work in the cold, but they should work again once it warms up.
Some materials will become brittle, and some things could crack from the expansion and contraction, but not electronic components (ICs, resistors, capacitors, transistors, diodes etc.).
Depending on the location, temperature and humidity, I would be more concerned with corrosion from moisture condensation and animals getting in, chewing on things and making a nest in my box of electronics.
Steve