I do not think that the microcode answer is correct, because this answer implies Windows not booting but the BIOS started Ok
As I understand the question, the BIOS does not start at all, or the BIOS start-up messages would appear on the screen.
Since you already replaced the power supply and did all the right tests, I am afraid that it is bad news, pointing to a motherboard fault.
I am bothered with the coincidence with the MS update, on the other hand.
A genuine MS update would have no call to go and play with the motherboard internal settings, but a fake update might.
Are you sure you you were on the genuine site, and that you went there by your own choice? There are a number of fake MS update site. If you followed an email, for instance, that would definitely be the case.
If your motherboad settings have been messed up, you should still have a BIOS startup sequence displayed, unless the screen has been redirected in the settings themselves (depends the PCs). If you have, for instance, a build-in graphic card on the motherboard, and a daughterboard graphic card (AGP for a 2 years old PC), try connecting your monitor to the connector for the built-in graphic port. The display could just be there.
If you have NO display whatsoever anywhere, starting with just a known working power supply, the motherboard, display, CPU, and memory, and still nothing happens, I am afraid that this is time to call HP to the rescue, because it it means a genuine hardware fault.