May be something has been failing over time and you just didn't know. Assuming you have spark and it starts with starting fluid, then you have fuel related issue. You don't have spark if it doesn't start with starting fluid. I was helping someone with intermittent crank and no start issue yesterday. He was able to start with starting fluid. It turned out his problem was bad fusible link. One of the circuits in the black box is blown.Unfortunately, this is where my knowledge wears thin - if I had to guess, I'd venture that I have spark as the truck has been running flawlessly since the day I got it (with only that one hiccup I mentioned above which happened for an hour, nearly two years ago), even taking it on a 200 mile round trip drive Sunday and it starting up no problem Monday morning when I had to move it out of the street sweeping zone... but that's the confusing part. No signs of anything wrong, even keeping with 15mpg on the highway the entire time.
That said, I will be heading to AutoZone tomorrow to pick up some starting fluid.
Other that that, any idea what could cause it to stop so suddenly? I'd always assumed that if a pump is failing, it'll happen over time and not on one day and off the next, literally 16 hours later with no signs and no use in between. I almost suspect that fact that since I filled the tank for the first time since June (my driving is very limited so 50-60 bucks to get me to 1/2 a tank every couple of weeks is normal), I'd inadvertently kicked up some debris in the tank which clogged the filter, but when I attempted to start it this past Tuesday, it didn't even sputter - it just wouldn't start in the same way it's not starting now. Nothings changed since the first attempt. So I just assume something died. But I have no idea what.
Also, I almost wish it was the relay because your bypass comes so highly rated and looks like such a slick piece of DIY engineering. I might actually pick one up anyway, to keep in my toolbox.
Thanks so much for your help