Just a guess: Is there an internal backup battery, similar to the one that used to be in the old G4 tower? If that dies, the computer may not boot (it happened to me with my G4 DP 450).

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365