There are 2 OSes: Ubuntu and Windows 10, why instead of choosing an OS, does Windows 10 start right away?

Question

Windows

I installed Ubuntu next to Windows 10, everything works fine at first, but as soon as I turn it off and on once, or just restart the laptop, Windows 10 starts automatically. The selection box does not appear. For a while, the OS selection window and then Ubuntu could also be launched through the boot menu (F12), options appear there:
Windows boot manager, if I'm not mistaken, and ubuntu. I choose ubuntu, the OS selection window appeared, I chose Ubuntu, the system was launched through Ubuntu. And now, at least choose ubuntu in the boot menu, Windows 10 still starts. Help !!!

Answer 1

1. Most likely, you have incorrectly set the priority in bios.
2. Then lilo/grub got corrupted.