What is the first thing you should do if you are communicating with another amateur station and hear a station in distress break in?
Why A distress call has absolute priority over all other communication. FCC rules (97.403 and 97.405) require any station to render assistance to a station in distress, using any means at its disposal. The immediate, practical step is to stop your ongoing QSO, answer the distressed station so it knows it has been heard, and find out what kind of help is needed so you can relay the right information to the right people.
Watch out Contacting an emergency coordinator or other authorities may come next, but only after you know who is in trouble and what they need; going silent or cutting power just leaves the distressed station calling into an empty band.
Answer first, then relay. Never leave a distress call unacknowledged.