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This is fairly common on a lot of cars, using de-icer spray is the safest way to temporarily cure the problem as hot water can cause parts to suddenly break, parts of the lock are plastic so it's not the best thing to use, the residue from the de-icer will also help prevent this happening in the future. However to help prevent this in the long term you can use a general purpose spray grease, this is available from any diy or auto parts supplied, spray it into the lock barrel, the latch on the door and if possible get it bit in the gap under the door handle to make sure all parts have a good coating of grease. Open and close the door a few times and then lock and unlock a few times to work the grease in. The grease will expell any moisture which freezes and causes the lock to stick. Using WD40 also works but it doesn't last as long as a thicker grease. Carry out this procedure on all doors and boot latch to ensure the problem doesn't occur again.
I hope this answers your question, please feel free to ask if you need further assistance, otherwise please remeber to "Rate" or "Accept" my answer. Thanks, Ross.