First thing: ask yourself, "Am I safe? Am I in danger of hurting myself or others?" If you're in such a state that your safety or that of others is in doubt, call emergency, call a crisis line, go to an emergency room - just get help. (online resources, PC Resources: Crises)

If harm is not in the picture, compose yourself (a whole topic in itself) - relax, take deep breaths from the diaphragm, rest, sleep if necessary.

"Do the next thing." Start, as slowly as necessary, to participate in everyday activities and tasks. Distract yourself. "Focus out."

OK, I've probably insulted your intelligence with all the above; nothing new there and all much easier to advise than perform. The important thing is that you remain safe. You matter regardless of what's going on in your life and/or mind.