Happend to me a couple of time on very cold day, like -25 celsius. Once I couldn't open the door after I left the X5 at the airport for 2 weeks on a cold winter, when I got back from Mexico, it was really cold and because I pulled too hard I broke the mechanism. I had a local garage to fix it for 200$ they had to order new mechanism + the door handle which was broken but they also had to paint the handle. When the door freeze I simply get in by another door, start the engine and let the truck warm up for 15 min and then the door open normally.