Never troied to drag it around, but you made me try.
I had the same problem with getting it back to place, I could drag it to above the message list, but not above the account tree pane. Next to the account tree pane was possible though.
I started TB from another Windows user account (pointing to the same message base) and the problem persisted, so the location isn't stored in the registry.
Therefore I decided to delete the tbuser.def file in the mail directory as that is the file where those settings are stored. Afterwards I started TB again and my menu was back in its original place.
However, all customisations made to TB (shortcuts, extra toolbars, etc) will be lost too as those are stored in the tbuser.def file too.

So the procedure is
Close TB
Delete the tbuser.def file in the mail directory
(that tbuser.eef when you're using OTFE)
Start TB

In case you've got a lot of customisations and a backup, you could restore the tbuser.def from your backup.
Procedure:
Close TB
Rename your mail directory
Start TB
Restore your backup
Don't collect any new mail
Close TB
Copy the tbuser.def file from your restored backup to your renamed mail directory
Delete your restored mail directory
Rename your original mail directory to the proper name
Start TB

Have fun