Open FTM and go to the Plan Workspace. There are 2 tabs, New Tree and Current Tree. Click on New Tree and then select "Download from Ancestry".
This gives you a list of the trees you have on Ancestry. If you don't want a sync'd tree unselect the "Link to Ancestry" and then click "Download" for the tree you want to move down. This will create a new file with everything from Ancestry but you will have lost fact notes and stories that were in your previous FTM tree.
If none of the trees are available for download because they were previously sync'd you will have to go back to Ancestry and break the Sync link at that end. Then, after a bit, the tree should be available for download. The data displayed in FTM will have to be refreshed so this can take a few minutes.
And now, instead of all this, how about just restoring that file from its Backup? You are doing backups, aren't you? If not, you should really make a point of creating a backup every day you use FTM. And have the backup stored off site, either with an online backup service or on a CD/DVD stored in another location. And don't overwrite the backup. Create backups with the date in the file name.