That means the power outlet is good. So you ruled that out. Now it comes to the power Plug or the Rachio that it’s plugged into. My bet is on the Power Plug, that is why you need a volt meter. You can get one for pretty cheap. Or see if a friend or neighbor has one you can quickly use. The power supply has a transformer in it. Transformers change one voltage into another voltage. In this case it takes the 120 AC Volts from the outlet, and changes it to 24 Volts AC at 1000mA. You can normally see this on the back of any of these type of power plugs. They all tell you what the output voltage is. So if you set the Volt Meter to read AC power. Put a probe to each wire, you should get around 24 Volts. Make sure you don’t touch those wires or that anything else touch those wires. You may be able to leave the wires plugged into the Rachio and touch the connectors and get a power reading that way. Or unplug the transformers, Pull out the wire ends about halfway so you can see the bare wires and yet they will be held in place. Plug the transformer back in, and then test for voltage. If you’re getting 5 volts or something low, or no voltage reading, it’s BAD and so the Rachio is not going to work as it’s not getting power.
I’ve had a number of these power transformers just flat out die on me. A Volt Meter is actually a pretty good thing to own anyway. If you have a Harbor Freight nearby, you can get a cheap one, or a little better one. I have a nice Fluke Meter, which I got half off when I was going to Trade School. $75 back then, over 20 years ago. Still have it and it still works great. I’m looking at harbor Freight right now and you can get one from about $23 up to $99. The $23 one would work just fine. All the others have a AC Amp Clamp on them. My FLUKE doesn’t have that. But I have those at work which I use all the time. Generally in checking 480 volt 3-phase motors. It’s one way to check for a Bad Motor. For general home use?!?!
If you are getting your 24 volts, then it’s the Rachio that has failed. This is why you want to narrow down what it is. You checked the outlet with a light as that’s all you have on hand and it worked. So that is ruled out. You didn’t have a popped Circuit Breaker or something. Now it’s what is after that outlet. Well it’s the power transformer, and then the Rachio. Just from experience, my guess it’s the power transformer. Those things are more prone to failure. But again, a guess is just that. A Volt Meter would answer that question in a minute.