Are you an ancestry.com subscriber (or Fold 3 or newspapers?)
FTM does not need to be registered to work, nor does it need to be online.
Ancestry only allows one computer to be logged into ancestry at a time. Do you have other devices logged in (laptop, other computer, etc?) If so, Log out (not just close the tab).
Can you go to File > Go Offline and start a new FTM file with one person in it? Exit and bring it up again? If so, after bringing it back up, try to go to File > Go Online.
Can you access your ancestry account with the same computer through a web browser?
If you still haven't been successful after trying the above, try their 800 number. Support isn't good with all things, but they are generally good at getting folks online and up and running.