Installed Ubuntu in parallel with the Windows but don't want to break Windows.

In superuser mode in live system:

mount /dev/sdX /mnt

# sdX indicates the partition where Ubuntu is installed (do not touch windows partition) .. to know about partitions of ur hard drive, use “fdisk -l” or lsblk command

chroot /mnt
grub-install --target=i386-pc /dev/sda
os-prober
grub-update
grub-mkconfig -o /boot/grub/grub.cfg
reboot

Remove live system media once the system poweroffs.

This should solve the issue.

If you are not able to make a live system, then from grub command line:

ls
set root=(hd0,msdosX) # you should know which partition the Ubuntu is installed
linux /boot/vmlin (press tab) root=/dev/sdaX
initrd /boot/initr (press tab)
boot

• X indicates partition number where Ubuntu is installed

This shoud resolve the issue.