My computer came with Windows 8 pre-installed so I shrunk the Windows partition to make room for Ubuntu. That it how it worked for the last year. After the second reboot in Windows 10 upgrade the computer did not boot any more. GRUB only displayed a command prompt. I found out later that the problem occurred because Windows somehow changed the partition scheme. The boot partition (containing normal GRUB data) was no longer where GRUB expected it. I don't know how and why this happened.