Do you have secure boot enabled in the BIOS? If so, it won't allow you to boot from another (non-Windows) OS.
Have you reset the BIOS/cleared the CMOS? If so, that would've restored the "secure boot" option in modern systems.
You should be able to switch from "Windows 8/10" to "Other OS" (or similarly worded option, depending on the BIOS vendor) and be presented with the boot options again.