Can't boot to my Windows 10 installation....

I have a Windows 10 installation on the same disk as my Windows 8.1 drive. A Linux installation is on the same drive and that can boot just fine. It's only Windows 10 that I can't boot to. Not even a doing a Startup repair solves the problem because startup repair doesn't seem to find the Windows 10 installation.

I am using the rEFInd boot manager but I can't get it to find Windows 10. Illustration attached to explain what the layout is. The EXT4 partition is the Linux OS. I am a bit out of my depth here - any tips what I can do?

Thanks Manish - screenshot below:

Strangely, the doing a boot repair from the Windows 10 CD didn't solve the problem.
However there is a solution and I found it myself:

Boot from the Win10 CD / USB. Go into repair options and choose to open a command prompt. Navigate to the drive you want to fix. Likely to be mounted to either C or D but could be other letters depending on the configuration of your machine. Double-check, you don't want to mess this up.

Navigate into the Windows folder on that disk. Write the command bcdboot.exe x:\Windows , where X=the current letter for the drive you want to fix. In my case it was D in this situation. When booting from that disk it is of course C.

After having done that, all you need to do is reboot. Windows boot manager will now know about this installation.