Changed boot order from HDD to USB, now it gets stuck on blackscreen and wont boot at all

Question

SOME BACK STORY:

I've had Ubuntu installed for a couple of weeks on my IMac and wanted to try out another distro. I made a USB with another distro, but I couldn't get to the grub menu by pressing Shift or ESC at startup. I have pressed every key on the keyboard but nothing works. I have also tried changing the /etc/default/grub file from (GRUB_TIMEOUT_STYLE=hidden -> GRUB_TIMEOUT_STYLE=menu) and changing a timeout thing aswell but nothing works I have also reinstalled Grub2. After a couple of days of "googleing" I found this page saying that Grub Customizer was an easy to use software that might be able change the boot order to the USB.

PROBLEM:

After downloading Grub Customizer I changed the boot order to have my USB as "number 1" and restarted the computer. When it started up again I was greeted with a black screen and an "underscore" (the underscore is not blinking). Nothing happens no matter what keys I press, I have tried getting to the grub terminal but the computer seems to be frozen.

This is what it looks like: Black Screen

When the computer was still working (running Ubuntu) this black screen was always seen for a couple of seconds before the normal ubuntu loading screen poped up. The computer does not make the famous Imac starting sound and haven't done it since I installed Ubuntu on it. It is just a completely white screen (with no apple logo) and then the black screen appears.

Answer 1

Perhaps your USB is not bootable, You may want to select boot from disc.

You can try:

Command (⌘)-R: Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the internet. macOS Recovery installs different versions of macOS, depending on the key combination you use. If your Mac is using a firmware password, you're prompted to enter the password.

More on that, visit:

https://support.apple.com/en-in/HT201255

Answer 2

Ok, So I got it working again.

I might have become a little bit mad at the computer and started pressing on litterarly every single key on the keyboard. I started from the numpad and I only got to the spacebar before the system started booting to ubuntu again. (I didn't have the USB plugged in). And no I don't know what key or keys made it boot again, the only thing i know is that it works again. :)