[Alan Anderson]

If the BAT and 6V LEDs are lit but the 5V LED is not, there are three reasonable possibilities.

The most common cause I've seen over the past few years is a sensor switch connected to the wrong Digital I/O pins, connecting the (-) pin to the 5V center pin instead of to the SIG pin on the other side. That shorts the 5V power, and the supply on the Digital Sidecar shuts down to protect the wiring. Disconnect everything from the Digital Sidecar and see if 5V comes back. Then reconnect things one at a time, and see if any of them causes it to fail. If you find the offending circuit, track down what's wrong with it.

The second likely cause is a bit of metallic debris among the DIO pins, also shorting 5V to (-) and causing the supply to shut down. With everything unplugged, use a strong flashlight to inspect the pins for anything that might be a problem.

The remaining possibility is a faulty Digital Sidecar. After you've eliminated the other options, then replacing it is a good thing to do.

[edit]Whoops, I misread which board was under consideration. I think my advice is still good, just not in this context.[/edit]