I was backing out of my garage and I don't think I went all of the way into reverse. I heard a pop, and now my car won't go into gear, and the clutch is loose.
Sounds to me that there is an issue with your slave cylinder. If it has a leak, either internally or externally, it will cause the clutch not to disengage. This can be determined by a mechanic who can inspect your transmission system and replace your slave cylinder if necessary.