Why did my wmv & mpg files used to play but now most of them wont?

How can I play my wmv and mpg video files that I have downloaded on my PC. I have windows XP and downloaded these files years ago and they all seemed to work fine. Now most of them do not even show the video in my folder, it only shows the wmv or mpg icon.I have paid money to download these movies off of websites and now they don’t work can anybody help me solve this problem so I can view all of my videos again.

Thank you