There are several things that may cause this issue. Try lubricating the lock cylinder with some WD-40 or equivalent light oil. Once lubricated well, use the key to work the oil in and try turning the key afterwards. If this doesn't help, you may also try a spare key, as sometimes the teeth on the original key can wear down cause it to lose the ability to turn the tumblers. More information about this issue can be found at the following web address .