How to choose a startup OS?

Question

ubuntu

The disk is divided into 2 partitions, on one is win10, on 2 I rolled ubuntu 15. When you turn on the computer, win10 auto-recovery starts, which does not lead to anything. I expected that there would be an OS selection menu. Ubuntu won't boot either, I insert a flash drive and press f12 and that's the only way I can boot ubuntu. When choosing an installation, they tell me that both wasps are already installed. What can be done to at least make Windows work?

Answer 1

Start Ubuntu from the stick, and reinstall grub.