I have a lenovo laptop with Windows 10. Some time later I installed Ubuntu on this laptop and could choose whether I wanted to use Windows or Ubuntu from GRUB. Today something strange happened, I turned on my PC as usual, but instead of GRUB menu I saw Lenovo Boot Manager with options Windows, Ubuntu and HDD, all of them sent me back to this Boot Manager.
I went to BIOS and set all settings to default. After that Lenovo finally loaded, but I don't have GRUB, Windows is just loaded like Ubuntu was't installed at all.
How can I restore GRUB or at least delete it together with Ubuntu to reinstall them later?