Your track may have disappeared from your profile because you were over your upload limit, or it was taken down for copyright infringement.
 

Over Upload Limit

All of our accounts come with a set number of upload minutes (check out our Pro page to see these limits). When you upload more than your allocated limit, your oldest tracks that don't fit will be hidden. If you visit your tracks page (https://soundcloud.com/you/tracks) you will be able to view any hidden tracks, these will be the oldest tracks in the list and will be greyed out.

Removed due to Copyright infringement

 
Whenever a track is blocked for copyright reasons, you should receive both an email and an on-site message to alert you. 

Follow the 'More info' link from your Tracks page if the track should not have been taken down, and you would like to dispute or learn more about the removal.