I asked this question a while ago but it got lost, so I deleted the previous one and created this new one with updates from the comments.
I just installed Windows 8.1 in a partition of my Lenovo Thinkpad T420 which also has an Ubuntu partition. As usual, GRUB was lost so I entered with a Live CD and recovered it succesfully, being able to boot both OSs.
The issue is that I already had a shared logical NTFS partition and I can't access it from Windows now. I entered Computer Management and the partitions are there but are shown with 100% free space. This is pretty obvious with the Ext4 partitions, since Windows can't access them, but this also happens with the NTFS partition, as shown in the following image.
I googled about this and ended up using DISKPART in order to manually mount my partition, but I got an error that says my partition is not assigned to any volume, and when I list them, no volume is shown for the partitions shown in the previous list in the following image.
If I run fdisk -l on the partition it says it can't be opened.

