I recently replaced a solenoid valve that was leaking. The good news is the leak is fixed. The bad news is that now none of the sprinklers work. The control system in the garage still appears to be working; I can start the cycle and the flashing light says it is running, but nothing is happening in the yard.

This has to be a water issue, right?

Probably a bad solenoid. I’ve bought and replaced brand new ones that didn’t work.

It can be tested but you'll need a multimeter.

Most likly you didnt get the ground wire back as it should be.
And did you turn the water back on, hey i made allot of money on that one over the years

What Bruce said.

Actually, yes I did check that.

The thing that worries me is when I shut off the valve in the first place, it was totally covered up with hard packed sand inside the box. I had to dig it all out to expose the handle to turn it. Then when I turned it, it doesn't feel solid. The stem is turning though.

Manually open a zone valve to see if the water is flowing.

Yea sounds like the stem on the gate valve might have broken. f the stem keeps turning, there is a way to fix it but will require more work,

picture of said valve would help

Follow Bruce's lead here. The shut off likely broke in the closed position, UNLESS you didn't wire the solenoid up correctly, UNLESS the solenoid your replaced was on the master valve and it wasn't actually the solenoid that was bad. A valve is both an electrical and a mechanical component and either one of those or both of those could be bad and causing problems.

My unit was only working on a few stations, the others - nothing.
Multi tester revealed a bad solenoid in the control box.

Hire Bruce.