There has been a few reported cases recently when a BIOS update has wiped the boot entry for EnOS.
First of all, go into your BIOS settings and check if it is actually gone.
If it is still there, you could simply change the boot priority.
If it is gone, as mentioned above, you would need to reinstall your bootloader from your live usb using arch-chroot.
Depending on what filesystem, the instructions vary a little bit. You could boot up your live USB and provide the output of the following commands:
sudo parted -l
and
efibootmgr
Copy the terminal output, paste it into your reply, highlight it and press Ctrl-E to format for better readability.
For a similar case, see: EndeavourOS bootloader missing after BIOS update (dual boot)