Ubuntu – How to open the boot menu

This might be a problem of broken grub. Grub is basically a boot loader. In other words it is the screen that comes up with options to start Windows or Ubuntu. I had a very similar problem. In my case I had dual booted Mint and Ubuntu. To fix the grub, check this.

Access boot priority menu

The boot priority settings are often found under the Boot tab of the computer's BIOS/UEFI utility. The BIOS/UEFI utility can be accessed by pressing the key which is shown on the manufacturer's splash screen when booting. This is the F12 key on many computers. The keys to press, such as F12, F2, Del or Esc, differ on computers from different manufacturers. When grub bootloader is installed it does not change the key which is used to access the BIOS/UEFI utility.

There is a table of keys used to access the BIOS/UEFI setup utility grouped by manufacturer and computer model at this answer.

Access GRUB menu

Immediately after the BIOS/UEFI splash screen during boot, with BIOS, quickly press and hold the Shift key, which will bring up a GNU GRUB menu screen. With UEFI press (perhaps several times) the Esc key to get to the GNU GRUB menu screen. Sometimes the manufacturer's splash screen is a part of the Windows bootloader, so when you power up the machine it goes straight to the GNU GRUB menu screen, and then pressing Shift is unnecessary.