tomkatz,
It is unfortunate that the changed artist wasn't noticed immediately - an Undo would have corrected the inadvertent change. But that's spilled milk...
The tags may still be available in iTunes - don't know for sure, as I don't really use it much. iTunes, like Media Center, manages both its own internal database of file tags, and updating the tags in the media files themselves.
Media Center by default writes tags back out to the files; if it has not already done this, you can ask MC to Update library from tags (right click tracks, Library Tools > Update Library (from tags). This will cause MC to re-read the tags, and populate its database with the file tags.
If that is not an option (because the Artist tags have already been updated), then you may be able to re-construct the artist tags if you have your files in some form of Artist\Album folder scheme, or possibly from your media file names. This may not work as well for compilation albums, but entirely depends upon how you location/name your media files. If you need to explore this option, please describe your media file location/naming scheme.
