Why have all my MP3 files turned into MPEG Layer 3 files as I installed Winamp today and had trouble installing it and ever since then none of my music files work?

Winamp may have installed incorrectly or been mis-associated, and either is trying to play them wrong, or damaged your music codecs.

MP3 is actually short for MPEG 2 Layer 3, so your computer is not technically mis-identifying them.

Try re-downloading Winamp, or downloading and installing a different player.