This is a copy of some one elses post but it may help. I found it searching under Ignition lock.
Here's the scoop:
Most likely, the tumbler is worn and no longer permitting you to turn the key. This is worse if you have a brass (replacement non MB key). If you do, get to a good indy or dealer and order a new key (or two) -- they are about $18. MB keys are steel, but do wear out eventually -- had to get a new one for the 220D this year.
Keep the MB key alone or with only a few other keys on a light ring, too -- a big bundle of heavy keys and other stuff hanging on the tumbler causes faster wear.
I'd also replace the tumber. Here's how:
Find a small hook or screwdriver you can get under the trim ring around the keyhole -- the inner one with the I, II, III marks on it. It's chrome, at least on all the ones I have. You will also need a 1.5mm dia stiff wire or jewelers screwdriver (available at Radio Shack, etc).
Insert key and turn to position 1 (steering unlocked, ignition off). Hook the trim ring and pull up to the key.
Turn key back to position 0 and remove key. Pull trim cylinder off. This reveals the lock slot/hole for the latch that holds the tumbler in place.
Insert wire or screwdriver, insert key, turn key to position 1, and pull key and tumbler out.
Put new key in tumbler (when you order a tumbler from MB by chassis # it will be identical to the original -- W123 and later chassis car tumblers are usually available from the US distributor and come with a new key. Older cars, and maybe some W123s will require ordering from Germany and DO NOT come with a new key).
Insert wire or screwdriver into new tumbler, turn to position 1, insert tumbler into lock, turn to position 0, remove wire or screwdriver. Tumbler will latch into place. Remove key and wire.
Replace trim cylinder, push down as far as it will go (it won't latch), insert key, turn to position 1, and push trim cylinder down.
You may need to remove the rosette, too, on some models -- it is held on the back by three bent tabs, and you may need to pull the instrument cluster out or take the bottom dash panel off. Not necessary on the W115, but I don't know about the others.
DO NOT wait until you cannot turn the key! You will have to take a dremel tool and grind out trim cylinder and tumbler -- takes my indy mechanic about 4 hours.