Shoutbox

If you have Messenger Plus! Live installed you can click the icon on the main contact list window then click Contact List Cleanup. Click the (Has You option at the bottom). Now this list has been having some problems lately and that is on Microsoft's end. But it should show contacts who have you and in this case if you know their email then you can re-add them from here.

The way I would recommend would be to go to Tools > Options > Privacy.

The contact will be on the Allow list (unless you blocked them too), so just look through that and you'll find them.