Laptop not booting from any bootable device (UEFI broken?)

I was trying to install Debian in a dual boot with the already installed Windows 10 on my laptop. Now when I boot up the PC it shows the Dell logo and the options to press F2 for Setup and F12 for Boot Options. It does not go anywhere from there, but that's probably because I screwed up the MBR.

If I press F2 in order to access the Setup, the screen tus black and shows an underscore [ _ ] in the top left coer. Then it just stays like that, and only responses to Ctrl+Alt+Delete.

If I press F12 in order to access Boot Options, it takes me to the BIOS Boot Manager. Here it tells me about the current "Boot mode", which is "Legacy; Secure boot: OFF", and lists the following things:

• Hard Drive
• Any other bootable or storage device I plug in (it detects all of the ones I have tried)
• Diagnostics
• Enter Setup
• Peripheral Device setting (OPROM setting)
• Change boot mode setting

I can select anyone of these and the following happens:

• Hard Drive = Black Screen, nothing happens (might be due to nonexistent MBR)
• Storage Devices = Black Screen, nothing happens
• Diagnostics = Same as above
• Enter Setup = Same as pressing F2 during bootup
• Peripheral Device setting (OPROM setting) = Here it tells me about the current "Boot mode", and then the option "Change Legacy Option Rom to:". Under that headline I think there is supposed to be a list, but it is not.
• Change boot mode setting = Tells me the current "Boot mode" and then the option "Change BOOT mode to:" with the options:
  • "UEFI Boot Mode. Secure Boot Off"
  • "UEFI Boot Mode. Secure Boot On"

If I change to any of the UEFI Boot modes, the following appears after reboot: Dell logo with F2 and F12 as options. After that it shows a black screen with the text "No bootable devices - strike F1 to retry boot, F2 for setup utility. Press F5 to tun onboard diagnostics."

All three buttons gives the same response: NOTHING. This text still shows. Not even Ctrl+Alt+Delete works.

I have not received any beep-signals.

Does anyone have an idea for how to fix this? Is there just some secret handshake that I have to do or is it totally FUBAR?

Sorry for a long post and thanks in advance for any help.

Laptop Specs:

• Dell XPS 12 Duo
• BIOS Version: A08
• Intel Core i7-3517U
• 256GB SSD
• 8GB DDR3