Beginner Gardening: Cactus Deflated??? Help!, 1 by SemiGreenThumb

SemiGreenThumb wrote:
I have 3 tiny cacti (1-4 in. high). They are all planted in the same type of pot and soil and sit in the same location, and are all on the same watering schedule. I have had the cacti for about 3 months with no issues. Suddenly, yesterday, I noticed that 1 of the 3 cacti is totally deflated...as if there was air filling it and now the air has been released. Any ideas on why something like this would happen? I am extremely concerned that whatever happened will also happen to other 2 cacti since I treat them exactly the same. I doubt that this cactus can survive, but I want to prevent it from happening to the other 2...Please HELP!!!

This pic is what my cactus looks like now...