1999 Other Mercedes Benz Models Car won't start/key won't tu

I have a 1999 Mercedes E320. The check engine light has been on for weeks. Three days ago I put the smart key in and it went all dumb on me. It wouldn't turn. I had the car towed and as it was over Thanksgiving, it was a few days before he looked at it. When he tried to start it, it started right up. He connected some sensor that was unplugged and th check engine light went out and life was good. I drove it to work and back. (8 miles one way) and when I got in to move the car to a different spot in the driveway, I started it and the check engine light is back on. Later, I tried to start it to go to work and the key won't turn. Again. Wht's it mean? How do I fix it and where are these sensor things so that I can plug them in myself/! Thanks, Bonnie