First, let me recommend reading information on the ignition lock in the DIY section.

This is not to avoid the question but to help you decipher where, and how serious, the problem can be.

Is the key failing to turn to the "II" position? This could be as simple as replacing the tumbler. To keep from having multiple keys, order a new tumbler from the dealer ~$20 (they will need proof of ownership).

If the key turns but the lock is staying engaged, beware of a possibly more serious problem. ($$$ if not caught in time)

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )