Frozen car door locks. Over my life, I've experienced this many, many times.
For an emergency opening (when you're in a damn hurry, and got to go somewhere), I've found pouring "luke warm" water down the side of the door where the key hole is will temporarily help open the door lock (never use hot or boiling water, as this could easily shatter the door glass).
NOTE: After you get the door open using the luke warm water trick, keep the door ajar by only latching the side door on the 1st click (door ajar mode). Never shut the door completely flush, as this will totally re-freeze and trap you inside the car from the damp door-jam rubber moulding that has now totally re-frozen. Run the car's interior heater at full blast to help deplete moisture in the door jam during your road trip. When you park at your destination, shut your door on only the 1st click (door ajar mode) and then lock your door manually with the key until your door jam area has completely thawed-out.
Preventative Maintenance is to frequently keep your door locks lubrcated with WD-40, and spray your rubbing door jam moulding gasket with a Silicone Spray (not WD-40). The WD-40 will help keep your locks from freezing and working smoothly. The Silicone Spray will help keep your door jam rubber moulding from sticking to the door jam metal. An alternative to the Silicone Spray is to use "Vaseline Petroleum Jelly" on the rubber door jam moulding as an anti-freezing treatment (cheap and it works great; kinda' like ChapStick for rubber moulding).