I know it goes against conventional wisdom, but my experience is rather sudden battery failure. When it happened on my Connie, I had just started the bike fine from home, went on a short ride, stopped to take in the scenery at the park, and then nothing. I thought it might be a fuse or something. That was with a conventional battery. It has a 0-maintenance sealed battery now. I'm not sure how that will work. The last time I had battery failure in a car, it was the same thing. Started fine in the morning then nothing when I went out to start it later.
I have seen batteries get weak and not turn over strong enough. That has usually been, in my experience, because there was something draining the battery.