My solution which worked for the problem I had (as described in the question);
- I booted using a live USB with Partition Wizard (free edition). The boot disk was listed as "unallocated memory" 100% free (which worried me a lot as there was important data on the disk).
- I used the "partition recovery"-function on the unallocated partition, and it was able to retrieve all the partitions, including my data (yay!).
- I was able to browse the disk using the "partition explorer", which gave me hope as I was able to see all my folders and files (I was uncertain whether they were corrupted though).
- Next, I booted using a live USB containing Linux Ubuntu, so I had an OS to access my files and copy them to an external HDD.
- After manually mounting the drive, I was able to access them and copy them to an external HDD. The files are not corrupted and 100%. It seems the bootmanager crashed and got corrupted at a certain moment. I was not able to restore this with the Windows Restore features.
- I ended up installing a fresh copy of Windows 10 after backing up my essential files (using Ubuntu as described in the previous step).
I ended up wasting what seemed like endless hours of work, but I guess it was worth it in the end as I was able to recuperate my most valuable files.