Retrieve a Sentry Turret to Defend Operations | Mission 4: No Hope in Hadley's A: CM Guide

Make your way back to the corridor which separates the medlab from the morgue, because that's where the disabled turrets are. Don't hurry too much as you'll almost certainly run into some additional aliens. Once you're there, approach the turret UA 571-C on the right and push the interact button to lift it.