Technician reported misapplying MEL procedures for an avionics cooling problem; resulting in the Avionics Blower Vent Valve remaining open; causing the aircraft to not pressurize correctly.

Narrative

Leaving work I was notified of an event that occurred on Aircraft X where the aircraft was unable to pressurize during flight. I had previously completed troubleshooting on this aircraft. During troubleshooting an avionics cooling problem; I was unable to duplicate the fault. The avionics cooling system can either recirculate its air or dump it overboard. The MEL must be properly applied to close the overboard valve. I do not recall any compounding circumstances during my troubleshooting; however I do recall maintenance being doing that interfered with troubleshooting. To begin troubleshooting I had to undo the procedures of MEL this allowed the valves to operate; once I had completed the extent of my troubleshooting I re-applied the 'M' procedures of the MEL as stated in my corrective action; I used a technical supplement along with the Aircraft Maintenance manual and MEL to complete this. While reapplying the MEL I remembered previous LMS about overboard cool issues and pressurization; as a result I verified and was very careful confirming the correct valve positions.