AN MD88 ACFT MAIN LNDG GEAR WOULD NOT RETRACT. ACFT RETURNED TO FIELD.

Narrative

I WORKED MAINT MANUAL SVC CHK ITEMS THAT REQUIRE MAIN LNDG GEAR OPEN (DOOR) AND CLOSE USING BYPASS LEVER. THE NEXT MORNING ACFT TOOK OFF BUT LNDG GEAR WOULD NOT RETRACT. AIR TURN-BACK AND FOUND BYPASS LEVER IN OPEN POS. I PREVIOUSLY CLOSED LNDG GEAR DOORS WITH BYPASS LEVER. I CAN ONLY ASSUME THAT LATCH FOR LEVER WAS NOT COMPLETELY LATCHED OR CAME UNLATCHED. KNOWING THE IMPORTANCE OF THAT ACFT SYS; I SHOULD HAVE DOUBLECHKED THE LEVER IN THE STOW POS. WHEN WORKING CRITICAL AIRWORTHY/SAFETY SYS; I NEED TO DOUBLECHK AND TRIPLECHK MY WORK. ALSO NOTE THAT PLT RPTED THAT LEVER WAS NOT OUT ON PREFLT WALKAROUND. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED ALTHOUGH THE MAINT GEAR DOOR HANDLE WAS FOUND IN THE OPEN POSITION UPON RETURN TO FIELD; THE PILOT DOING THE WALKAROUND STATES THE HANDLE WAS IN THE CLOSED POSITION BECAUSE THAT IS ONE OF THE ITEMS HE CHECKS FOR ON HIS PREFLIGHT. THE OTHER POSSIBILITY IS THE HYD SYSTEM WAS SHUT OFF PRIOR TO FULL DOOR-UP CLOSURE AFTER OVERNIGHT MAINT WORK COMPLETED.