Reactivate grub/edit boot partition used separate partition.

I used to have windows and Ubuntu operating system installed on separate partitions where I had grub and I could choose the operating system. Now after I reinstalled windows, grub does not come up anymore.
I still have Ubuntu on the other partition.
Is there any way to choose the partition and I want to reactivate grub/edit boot partition?
What is the way to do I can boot into Ubuntu instead of windows?
