When you say 2006, do you really mean FTM version 16? There is a big difference between FTM version 16 and the current versions 20XX
If it is FTM version 16 try importing it into the the 200x version.
In 2012 you would go to new tree from the Plan. Then from the File menu you would select Import as New Tree, then find the directory where the old file existed.
Since you have worked with your file some, I hope you have been doing it on a back up because you may have corrupted it in trying to open it.