Ditto. Probably had a cell or 2 crap out. It can happen just that easily. New battery time. Then take note of how it acts afterwards - ie: Does it eventually crank slow, etc which could point you to some sort of parasitic draw that is slowly killing it.
Loose connections on the cables or battery terminals can do that too.
I stripped out the bolts on my battery clamps. I used toothpicks to give the thread some bite to get me to work the next morning. Worked, but eventually loosened up. Started and drove. Went a mile down the street and parked at the burger joint. Went to leave and nothing! I was able to hand tighten just enough to start and get home, but the truck was dying as I pulled into the driveway. New clamps and tight connections fixed for good.
Test the battery, but check the cables at both ends to make sure connections are tight and grounds are solid. Clean grounds, battery terminals and clamps while at it.