Mp3 files can't be opened anymore after editing some tags

I downloaded several musics from YouTube and was able to listen to them normally, but when I used mp3tag to change some tags, they suddenly became unavailable. I tried using mp3val to repair them but it didn't work (it said it worked, but they still can't be opened).
What can I do ? There are about 50 broken files and I don't really have the time to download them again

You could start with this instructions with the title:
How to repair files identified as corrupted?

Odds are these aren’t properly formatted. See here for similar issues.

Just to be sure: the files themselves are corrupted thus you cannot listen to them anymore - and not the data in theirs tag fields?

As for downloading from YouTube: how did you do that? I both both curious what method you used - but also I would like to advise you to use from now on the YouTube Video and Audio Downloader (Dev Edt.) add-on for Firefox [see info at YouTube download in high quality]