Its unlikely that the spring just popped out. Unfortunately its more likely that one of the mounting points to which the spring attaches snapped and broke, and is now gone. That's why you can't figure out where to attach the spring. If you look carefully you will see the part that broke.

I had a similar problem with my mirror, I bumped against it accidentally, , and instead of it folding out of the way, I heard a snap and the housing broke. The spring came flying out. Bought a new mirror.

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'95 E300 Diesel, 264,000 Miles. [Sold it]