Try this:
- Unplug the PC (PSU).
- Then press the Power Button for at least 30 seconds.
(the on board ROG LEDs will sometimes flash after 20-30 seconds to show that all power has been drained from the board).
- Use a flat headed screwdriver to short the CLR CMOS (CLRTC) Jumper (Touch both jumper pins with the screwdriver) for at least 15 seconds.
CLRTC jumper location can be seen on page 1-2 here:
https://dlcdnets.asus.com/pub/ASUS/...ING/E14401_ROG_STRIX_B450-F_GAMING_UM_WEB.pdf
- Plug the PC back in.
- Press the Power Button to turn the PC on.
- Repeatedly tap the DEL key
- A screen should come up telling you to press the F1 key, do that.
- Set the Optimized Default settings and Press OK (this is on the EXIT tab)
- Save and exit the BIOS.
- Tap the DEL key again to enter the BIOS and change any settings you want changed then save and exit again.
I sometimes had to do it a few times before I could get it to boot again.