OK, this took a while to figure out. But, I've had a Rega P3 turntable that is plugged into a Rega Phono Mini that suddenly stopped working. It was strange the LED looked on but no sound came out. Finally, the hint for debugging is that a phono preamp amplifies the low signal generated by the mechanical movement of the stylus and applies RIAA equalization.
So if you want to test a phonograph, plug it directly into the receiver and carefully turn it up. Then you should hear something which I did and it worked.
Turns out that both the phonograph and the preamp use a small 12V wall wart and the barrel plug got loose so the trick is to strain relieve the plug and then plug it in gently and it works!
Theoretically, Rega has a lifetime warranty, but when I went to the local dealer, they would only fix the barrel if I had bought it there and I have no idea where the installer purchased it. Theoretically, you can get help from their US distributor, but good luck with that and there is only a three-year warranty. And apparently, it would cost more to repair than to just spend $199 and buy a new one.
But then it probably makes sense to get to a slightly more high-quality phono preamp. You can definitely spend a fortune on them.