Our Garmin GPSMap 392 did something similar last fall. On the way to haulout: No GPS. It would show only one satellite acquired and a bunch of others there, but not acquired. I think at one point it acquired two.
So this spring, before embarking on a New GPS Quest, put a battery aboard and tested the GPS. Worked right off, no problem
Gonna have to test it again.
We're thinking a backup GPS would be a Good Idea.