No Boot Device Found.

If the problem is what the message states, turning off Secure Boot should have allowed it to boot.  You do not need to go to Legacy since a UEFI system will not boot that way.

If something is corrupted, hard to tell what type of message you might get.  I have seen a bad drive cause problems because the system cannot read it.  But your system should be able to boot to a DVD or USB drive is configured correctly.

Someone may tell you to run the boot diagnostics to see if the drive is having a problem.

Please run the Diagnostic tool to check hard drive on the system by following steps below:

1. Power off system

2. Power on system, quickly tap the F12 key continuously until you see Preparing one time boot menu in the top right corner of the screen. In the boot menu, stroll down to Diagnostic and then click it. After quick tests, click on Hard Drive and then Thorough Test Mode.  Report back any error code.

Also check and see if the Hard drive is detected in the BIOS

If there isn't any error code and Hard drive is detected in BIOS, Reset the BIOS to Default Values and see if that will make a difference. If the problem still isn't fix, I suggest you download Windows 10 installation media in the link below to either reinstall windows 10 or use the Repair Your Computer option.

Download Windows 10 - microsoft.com

nvm it worked. No problem was detected after running the diagnostic so I guess I need try what LSUFAN51 suggested.

when I try to run the diagnostic, the computer crahses. I get this colorful bars on screen so I have to turn it off. Do you have any ideas to what that means?

When you see the colorful bars, you must quickly click "Yes". If you do not respond for 30 seconds, the diagnostics will automatically assume your response as 'No' and proceed to the laptop LCD built-in self test (BIST).

I ran the diagnostic tests but it crashed. A colorful bars came up so I had to turn it off. It's still stuck on No Boot Device Found loop

nvm it worked. No problem was detected after running the diagnostic so I guess I need try what LSUFAN51 suggested.

So when you start your system and use F12 to open the boot device menu, you do not see a Windows Boot Manager you can select?

Reinstalling, will usually clear up problems with an install and you probably already knew that.  But something messed with your normal boot.  If you get the system back, be aware if the hard drive is the problem, you may want to back everything up.  If it was the Windows update, then it might happen again when you receive the same type of update...

Your situation is the first time I have seen a message regarding Secure Boot capability, on a system that was booting normally.   I believe I would be interested in a possibility of a virus infection..

Well, in this case first scan for the malware or viruses as this also cause the error and after that enable- disable the fast startup feature. Also check the install graphics drivers. It is found that this is a graphics driver issues particularly for the laptops that are switched between Intel HD Graphics and from Nvidia or AMD. So verify this and disable your AMD or Nvidia graphics driver.

http://fixwindowserror.weebly.com/

Try this worked for me

Operating System Loader Has No Signature . . .

these steps resolved my issue:

1. On Startup, press F12

2. Reset Bios to default settings

3. Restart laptop

4. In Bios, select Boot Sequence menu

5. Manually ADD additional line with the name "Windows Boot Manager"

6. Select /EFI/BOOT/BOOTX64.EFI

7. Exit

This helped with my gf's computer. After she installed one update today, the laptop couldn't find no bootable device, even if SSD was recognized. So thanks @Mansoorpataudi, after 4 hours of searching, your finally did it

Thanks for this!  Dell support told me to re-install the OS.  While I was on chat with them I tried your steps and they worked.  I shared your information with them.  How did you figure this out?

Thanks a bunch! Been searching for awhile on how to fix it and your suggestion worked.