You need to change the default player for those files back to media player. Here's one way to do it.
Just go to the LW library page hi-lite the folder with your files in it and click the "Explore" button.
The folder will open, then just right click on one of your files, Click "Properties", then where it says "Opens With" click "Change", then select the player you want from the list and click OK.
This should change most of them back but just repeat for any files that didn't change. You may need to restart LW for the changes to show.
More info:
Setting Default video app