I had a dual boot windows7/Ubuntu on my PC and it worked fine. I then installed windows 10 on the windows7 partition. The problem is that I have overwritten linux grub and now only the windows OS is functionnal, yet the linux partition is still existing. How can I repair the dual boot?
You need to boot into a live distro such as a ubuntu, or linux mint session from USB. From there you can reinstall grub.
This will go into more detail: