if my guess is correct, yours is a desktop. r/r memory is simple - open up the box, find the memory sticks, press end tabs, it should pop out. when you install, make sure align well, then press down.
But your cause may not be that of memory. if the technician just repaired it, s/he should make it boot up.
One thing - double check if power cords have been plugged to box, monitor and wall outlets.
