Yikes, is the crown:
1. loose
2. or broken? (aka snapped)
If it's loose, get some glue, open the caseback, try slowly to 'move' the stem out of it's place. Then apply some glue to the crown and thread it back onto the stem. Voila!
If it snapped get yourself a new:
1. crown stem
2. glue
and you can put it back together... not sure if the gen crown stem will fit. I heard somewhere that it might... experts on this?
I do not open watches myself, but I watch my watchmaker as he does it
Hope this helps.