I got a Mac OS X virtual machine on VirtualBox and when I started it, I got into this UEFI shell:
What should I do?
I also hit the problem of getting stuck at the UEFI shell prompt when booting from a viable disk (such as my case where MacOS had just been installed in a Virtual Machine).
I was able to fix the UEFI problems as follows (credit to VirtualBox forum):
At UEFI prompt: Type exit
You'll be brought into an EFI text-mode GUI.
Select Boot Maintenance Manager and click.
Select Boot From File and click
You should see two entries in a list (they are cryptic looking PCI bus paths).
The first is the PCI path to a boot partition that isn't working (corrupted or perhaps has no data or OS installed on it).
The 2nd partition is the recovery partition, the one you need to boot from to do the macOS installation or other maintenance/administrative tasks (including accessing Disk Utility to work on the System Volume partition without an OS running).
Click the 2nd entry, you should see (and then click):
macOS Install Data
Then (if present), click
And finally click:
At that point he installer boot will continue and you should get to the point where you can attempt to repair the volume or [re]install macOS