Oh yes!! We all have days like that. I remember going to my favorite park in Utah and finding nothing at all. Going back another day and find silver after silver and wondering if they just came to the surface!!
I think a few reasons account for this. Sometimes our mindset. Thinking about life instead of concentrating on the signals, or hearing the noise around you so you can't concentrate. Or just plain not scanning where the coins are!!
Anyway you got out, got some exercise, was probably able to relax and think of nothing, and THAT is almost worth not finding anything.
I repair computers all day and when I get under the water I don't have to think about anything, relax and hear the beeps!! Good luck and keep trying. It is a great hobby to get away from the world.