Step One. Insure that everyone is moved out of harms way.
Step Two. Insure that the area is marked so that nobody enters
Step Three. Now is the time to try to look to stopping the collapse. And how to stop it always depends on where, what.
The load is replaceable, people are not and are the only thing that matters.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.