Wow... lots to do.
I made sure there was no water running in the house, looked at the valves on the backflow assembly (both fully on), then checked the meter at the street. Perfectly still. No movement of the little triangle or dial.
Pipe break: This is scarry... but again, the pressure in the house seems to be the same as before. I took a shower with no difference that I could note. I've done dishes by hand with lots of pressure from the faucet and sprayer. Toilets flush and refill just fine... I don't know where to look for "master valve" and I sure hope if I have one, it's not broken.
I examined the backflow assembly again. On one side there is only a pipe coming up from the ground to the assembly (presumably this is from the main water supply). On the other side, the assembly is connected directly to a pipe which exits the box and goes into the round box with the meter. The pipe then exits the meter and goes out the other side of the round box into another box that has lots of wires and a solenoid (at least that's what I think that thing is.)
Someone running around my house turning down all of my sprinkler heads (there are at least 40 of them) is unlikely, but I do have cameras outside. I'll check the recording (if it hasn't re-written over last Tues. night) for any mischief.
The valves at the backflow assembly only make a quarter turn. There is a stop at the full on position and the turn valves are both resting on the stop point. I believe that they are completely on. Also, I did open the valve coming to the backflow when I had the housing open to see if water flowed freely. It did, and quite forcefully. What happens as it leaves the backflow is a mystery. Maybe the exit is somehow stuck closed??? Some water does get through because my sprinkler heads do bubble. They just do not raise and spray anymore.
Bad karma, I think. I'm going to break down and call the irrigation people. I have to get this fixed before I go on vacation later next week. Otherwise, all of my flowers and plants will wilt and die in this awful heat (Alabama in the summer... got to love it.)
Thank you so much for all of your thoughts and helpful ideas. It's been great having someone to talk to about this. I've learned a lot. (Mostly that I need a professional.) I'll post again after someone figures this out.