Do you have VLC player. If not get it here and try it out.

www.videolan.org

If it still does not recognize them after installing VLC try rightclicking on a file and choose open with then press VLC (you may have to browse for it if not on list), check "always use..." and then press OK

If it still does not react, try using VLC to browse for the file and press open.

If still not responding then it's a more serious problem