To be clear: I said System Restore, not reset or refresh. System Restore would just roll back system changes to a date when you said things were working, if such a restore point is available. Unless you have made a lot of major changes to your system (your personal files should remain as they are right now) then this would be hardly noticeable (other than, perhaps, restoring the ability you said you once had to play these files on this particular computer.)
However, because of the way System Restore and restore points work, you have limited time to try this easy and non-invasive solution - as the restore point you need (yesterday? Day before that?) will disappear soon and that option will be gone.
i have hundreds of different types of video files and have had no trouble playing them on Windows XP, 7, 8.1 or 10 TP as well as OS X of various versions and Linux of varying distributions... If this ONE system was working and suddenly stopped, if the amount of time between working and now is short enough, a system restore is non-destructive and could undo whatever change happened that made the playback stop working without much effort.