In reply to Luciano (StackExchange wouldn't allow me to comment, so I must put this in an own anser) - I had the same problem today and will write here the exact steps for testdisk...
- Choose disk (/dev/sda in my case)
- Choose Intel
- Choose Advanced
- Choose with up/down arrows the correct partition and with left/right arrows the Option [Boot]
- Choose [Rebuild BS]
- Testdisk complains that "Extrapolated boot sector and current boot sector are different." - Choose [Write]
- Confirm with Y and Quit
After reboot into Windows-Recovery using "starting-help" (or whatever it is called in English), it said something like "Disk is being repaired, this may take over an hour" - but it was finished after about 20 seconds.
And yay! I can now boot into Windows again, AND the NTFS-filesystem covers the entire partition. So thanks again, Luciano for the excellent suggestion.