I guessing that you meant to include the word 'on' after 'the ec light'.
The most common cause of the A/C compressor not engaging is low freon charge. The second most common is a faulty A/C temp/pressure sensor.
There is a built-in diagnostic system for the A/C controls. Info on how to access it and interpret the results can be found here -
AC Diagnosis | EClassBenz.com
Generally, A/C is not a DIY project since special tools and knowledge are required. If your A/C was working before you probably have a leak. An A/C tech can find the leak, fix it and pressure test before adding the correct amount of freon and oil.