My Check Engine Warning Lamp came on today. I have a 1998 CLK320 with a little over 49K miles. Any insight as to what causes this? Just had a "A"service a few weeks ago. Would appreciate any feedback from anyone who has had similar problem including cost of service to repair. thanks

There are many components that would cause that. Most common on that vehicle are faulty oxygen sensors and mass air flow sensors.
Don't sweat it, just get someone to run the codes for you.