All I can hope is that there's a solution, like uninstalling VLC, trying it again, or another player ...
Maybe another player would be in order, there is likely one or more on the Software Manager to try. Am not on a Linux system at the moment, though here's a good place to start.
And if all fails, you can reinstall your prior distro where your videos worked after back up of your files, preferably using two methods. The inbuilt backup tool (or Timeshift & Aptik), plus using an external, create a folder (a NTFS formatted drive is fine), and copy all of the sub-folders with content inside of your main Home folder into the one on the external by using the drag & drop method. I prefer that over any backup method with Linux, because post install of the new OS, can delete the like sub-folders of the new install where there is a like folder saved & copy the same folders back, one by one. Depending on how much data is stored, plus whether it's connected by USB 2.0 or 3.0 (or eSATA/USB 3.1), will determine how much time it takes to copy to an external & back over.
If you want to save your bookmarks/extensions, simply follow any instructions on how to create an account & you get to keep these also. Firefox makes this painless (about 5-10 minutes total), so does Google Chrome, plus you get the benefit of 15GB of free storage. Which is valuable when used with the Insync client when on promo for free. Dropbox users also gets 2GB of free storage, not a lot, yet great to have for when in a jam.
So if needed, you can revert back to an earlier version of Linux & lose nothing, the same applies to new clean installs, as well as future upgrades when one wants to keep their files, yet install cleanly. Done properly, one loses nothing, which is good for all.
Note that I didn't learn these things overnight, and I don't expect anyone else to. Though I learn a lot simply by the reading of articles, which I do most every day.
I feel confident, that no matter what you have to do, you'll regain access to your files, even if that means moving away from Ubuntu MATE. However, if you otherwise enjoy the OS, I recommend that you take a look at the link above & try a few extra media players. While VLC is an excellent media player, and far more capable than many, it's not the only one available to you. Because with Linux, you have choices.
Please keep us informed of your progress & when you find the one that works for you, let us know which one done the trick. A little hint, out of that list, XBMC (now Kodi) is likely while among the top media solutions, is also the hardest to configure. I've played with it, was wanting it to be a replacement for a TV, yet no success. Admittedly, it's me likely not knowing what I'm doing.
Yet Kodi should be much user friendly to configure, regardless of which OS one is using, here's some tips if you'd like to try, as well as requirements, that I just now discovered.
Though keep in mind other other than VLC & Kodi, there's 8 other options on the list.