my video files wont play anymore

I downloaded an i pod mp4 format video and i tried playing it. it wouldn t work because i dont have and mp4 converter but furthermore my problem is that for some reason all my regular format videos such as mpeg avi and so fourth wont play in my media player or nero showtime or anything. Visualizations play with the audio before everything worked fine. Can anyone please tell me what went wrong or help fix please. Would be greatly appreciated
thx

Are you saying that .avi files played before but not anymore?