Sometimes grit can be lodged inside the lock barrel and stop the key from turning. This is uncommon, but the following method usually fixes this problem.
You will need to use a spray lubricant such as WD40 and spray into the key hole. Insert the key and jiggle around, take the key out and repeat the process a couple of times. This will usually dislodge any blockages within the barrel and will allow you to open/close your letterbox.