my guess is the sunroof drainhose froze and popped off. pull down your headliner in the corner and see if it's attached... it's a relatively quick/easy fix.
1992 BMW 325i (CHUMPCAR #296 - build in progress)
1992 3.0L M52 325i (ex-daily driver - track car only)
2002 330Ci (ex-daily driver - sold!)
Check out my e36 track car build