Here's what happened...
I made a dual boot system with Ubuntu 16.04 and Windows 7 (with a GRUB boot loader) on my 1 TB HDD allocating about <100 GB (including all proper partitions) to Ubuntu and 200 GB to Windows (since I read that it's better to install the OS to a smaller partition) leaving the ~700 GB of unallocated space, thinking I could partition it later since I was busy at the time.
The system ran perfectly fine until I decided to partition all of the unallocated space using Disk Management on Windows 7 (probably not the greatest idea ever). It took some time, so I went on to do other stuff. Next time I checked, I saw an "endless" stream of command lines such as:
[ 39.353475] Out of memory: Kill process 684 (systemd-udevd) score 119 or sacrifice child
[ 39.353549] Killed process 684 (systemd-udevd) total-vm:520776kB, anon-rss:494072kB, file-rss:2560 kB
until a blank screen appears.
After restarting the computer, I tried booting Windows and it hangs on the "Starting Windows" screen. Trying to boot Ubuntu results in the same "Out of memory...." failure.
Safe mode with networking doesn't load either, gets stuck on "CLASSPNP.sys"
After this, I've tried connecting the HDD to an another computer with Windows 7 (since I only need the files). Strangely, whenever the HDD is connected to the other computer, the Windows 7 from the other HDD also hangs on the "Starting Windows" screen. After disconnecting the HDD, other computer Windows 7 loads perfectly. I'm absolutely sure that the first boot device selected in BIOS was from the healthy computer.
Only thing I need is to backup the data stored, I don't think I'll mess with any dual boot systems for a while after this. :D
Any help would be greatly appreciated!