It may be possible that some debris/dust is caught in the lock of your unit. Please try the following:
1. Check the engraved key number and verify it matches the engraved letter on the key cylinder.
2. Spray WD-40 oil in the lock.
3. Tap it lightly with a hammer.
4. Try the key again. If the key still does not work please contact Consumer Affairs at 800-323-9005 for more information.