A hard reset and restore did not help because you just restored back the problem.
I just had a look in WMP but could not find anywhere to modify file associations. Unless someone knows of a registry hack, you may really need to do a hard reset without a restore to get it back. However, one way around this may be to open files in File Explorer, then when it prompts to choose a program with which to open, choose WMP.
Moved to Multimedia.