Crown rot happens to semps with sunlight/water/temp shocks & is usually deadly. You MAY be able to nurse that 'Black Prince" through by trying these ideas:

1 - Put in shade (bright is fine - just out of direct sunlight) & do not water.

2 - If there are healthy leaves left, carefully pull off the soft ones & make sure there is very good air circulation to what is left of the plant.

I hope somebody else has discovered better first aid & will chime in here, but hope this helps! marea