I agree that a little more detail would be helpful.
Is it just not producing cold air or is it completely dead? There are built-in diagnostics but the unit must at least be active to initiate the tests.
Details can be found here -
W210 Air Conditioning Diagnosis | EClassBenz.com
If it's dead, then about the only things to check are the connecting cable and the fuses. To pull the unit and look at the cable you'll need a set of 'radio keys'. They go in the slots at the bottom corners to release the locks for removal. Available online, at parts stores and from a dealer. Dealer (OEM) work the best in my opinion. Did you check the fuses with a tester? Eyeballing isn't always reliable. The two important fuses are #4 and #7 under the hood. There are usually spare fuses stored in the empty slots of the fuse panel. Since the 'joy to cost' ratio is so high you might want to just switch fuses for a test.
If the unit is getting power and is dead, chances are it has failed. There are companies like Beckman Technologies to do repairs/exchanges. New CCU's are very expensive so a replacement probably only makes sense on an older car.