....but you CAN undo it. You have an edge case situation in not remembering the track you Thumbs Down and NOT having a Premium membership. ALL edge cases can't be accounted for in all situations, especially in a standard account. As stated all the effort you are putting into a single track that you don't even KNOW you are missing is futile. Here are 2 things to consider.
1) Since you don't know the track maybe it was something you DIDN'T like or indifferent to.
2) How about assuming that Elon is correct and Spotify is coming and may replace Slacker all together. Now you can start all over without any effort and you will have your unknown track back.