I was doing the drowned sorrows quest and after gtting the note and going back to Winterhold Ranmir wasn't there. I looked for him in the Inn and it's cellar then in Birna's shop. he wasn't anywhere.And I know he didn't die from any other quest like containment or him dying by a dragon or whatever. (I'm playing on X-box 360)
He can die really (really) easily he could have died out of sight and bodys are sometimes cleaned up pretty quick. Leave the area for ten in-game days and see you can find him after that but theres not much you can do on a console.