1. Get people to safety by following the floor warden's information and get out if needed
   If there are no floor wardens, YOU take the initiative and alert people. Vacate the premises if you suspect danger.
   Err on the side of caution. Nobody ever got fired over keeping people safe.
2. Get people to safety (yes! double check this)
3. Check what is happening
4. Stop the bleeding
5. Check what you broke while stopping the bleeding
6. Check if you need to go into DR mode
7. Go into DR mode if that is the fastest way to restore the service
8. Only now start to look deeper

Notice what is missing in this list?

• WHY did this happen?
• WHO did what

During the first reactions to an event, stick to the facts of what is happening and the symptoms. If the symptoms are bad, attend to people first, no matter the financial losses occurring.
Remember that financial losses are typically insured. Human life is not. Only loss of income and ability to pay is insured! Not the person's life.

The WHY, HOW, WHO and other root cause questions are asked in the aftermath of the incident and after you have stabilized the situation.
In ITIL terms, those are Problem Management and Root Cause Analysis stage questions.