I have a side-by-side installation of Windows 8.1 and Ubuntu. As I am a pretty helpless user it took me some time to set it up, but it worked and when I start the computer, I get a choice between running the Ubuntu or, if I type "esc" and enter "exit", it jumps to Windows and starts Windows. Today I was asked to update the Linux files. I did, and when I returned after a minute or so Windows was up. ???? I restarted the computer, but now only Windows starts. It is as if Linux never existed - in fact something prevents that the system asks for the choice. Is there something I can change to recover my Ubuntu partition? After I tried to repair GRUB (as given in comment below) I get the following boot loader menu, and I include what happens when I choose any of the options:
Ubuntu => black screen, caps lock blinking, nothing happens anymore no matter which key I press
Advanced options for Ubuntu => same without caps lock blinking
Windows EFI bootmgfw.efi => Windows starts regularly.
Windows EFI Boot UEFI loader => Windows starts regularly
EFI/Ubuntu/fwupx64.efi => the boot loader page is entered again
EFI/Ubuntu/mx64.efi => blue screen “shim UEFI key management” appears, when I press “any key to continue”, another menu “Continue boot/enroll key from disk/enroll hash from disk”, when I choose 'continue boot', black screen as above
EFI/Toshiba/Boot/bootmgfw.efi => black screen as above
Windows Boot Manager => Windows starts regularly
System setup => "error: can’t find command 'fwsetup'. Press any key to continue" => back to boot loader page.
When I start Windows (8.1) and go to the Windows Boot manager, under "using a device" Ubuntu is listed as one option. Choosing this option leads again to the Ubuntu boot manager, so nothing is gained.
The boot repair diagnosis can be found at http://paste.ubuntu.com/p/qv42scvngD/