If you do not get a GRUB menu , __re-installing__ or __repairing grub__ will NOT HELP.

1. Go to BIOS settings (press <kbd>Shift</kbd> while booting )

2. Using arrow keys, select __UEFI Boot Option Priority__ ( or _Boot option menu_ for systems)


3.  Usually Windows is the default , select elementary  on the UEFI boot menu you will then get a grub menu.