Happened to me last Thurs.

Been monitoring battery voltage since I been seeing the battery sides expand. Cells shorting out because of very bad roads, high temps are the killers on my batteries. I noticed voltages at the terminals went from 12.64, to 12.40 then 12.20 in about 3 weeks. Till all I got was a click when I turned the key last thurs.
Luckily I was prepared with a new battery on the side and changed the old.
Then changed out the terminal yesterday when I got a click again last Saturday. New bat voltage was still at 12.64 at the post. Wiggled the pos cable at it started.
All good so far new bat and new pos terminal.