See if any of these links are useful:
https://www.disk-image.com/faq-bootmenu.htm
https://www.howtogeek.com/131623/how-to-clear-your-computers-cmos-to-reset-bios-settings/
https://www.wikihow.com/Reset-Your-BIOS
https://www.lifewire.com/how-to-clear-cmos-2624545
In case none of the above work then consider contacting the motherboard manufacturer customer support.
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