Hi @hugetree
Try a power refresh in case it is a corrupted BIOS.
Turn off the PC and remove the power cord from the PSU.
Open the desktop and remove the coin cell RTC (aka CMOS) battery from the motherboard.
''Press and hold the Power button for a full 30 seconds and then release it.''
Reinsert the RTC battery (usually +ve on top as marked on the battery)
Reassemble the desktop and connect power to the PSU, turn it on and check.
If still no good connect a [https://www.ebay.com.au/itm/374429389685?chn=ps&_ul=AU&norover=1&mkevt=1&mkrid=705-139619-5960-0&mkcid=2&mkscid=101&itemid=374429389685&targetid=&device=c&mktype=pla&googleloc=9070553&poi=&campaignid=15791083372&mkgroupid=&rlsatarget=&abcId=9300816&merchantid=548727045&gclid=EAIaIQobChMIg6qWxOW0_QIVSiQrCh34OgQ7EAQYBCABEgLqq_D_BwE|system speaker] (example only) to the Power LED and Speaker Header pins as shown on p.28 of the [https://download.asrock.com/Manual/B450M%20Pro4.pdf|user manual] and check if there are any beep error codes heard. (see image below)
[image|2875159]
(click on image)