At Wits End

[tcowles]

I have spent at least an hour looking at old posts concerning "No Start Problems". None seem to fit my problem. Here is the story.
Two weeks ago I purchased a 1989 190E, 2.6. The car has 143 K on the meter and cosmetically looked pretty good. I noticed several small problems (most from neglect) but at a 2000 price, I took a chance.
For the past week, the car has been in the repair shop. My mechanic replaced the spark plugs, plug wiring, distibutor and rotor. He started the engine and said that it ran great. A half hour later he tried to start the car again and the key would not turn in the ignition. The steering wheel is also locked. He tried most of the things I have read on this forum to get the key to turn. There had been no previous indication that the ignition switch was going bad.
I am hoping that some one with a similar car can shed some light on this problem. Are we missing something other than the fact that the ignition lock suddenly went bad.
This car is equipped with a factory alarm system. Is that acting up?
Any ideas will be welcome.
tcowles, Inverness, FL