If your PDC went off-line before promoting a BDC, the registry entry on the PDC which governs the server role was not changed. When you bring the PDC on-line, it comes up as a PDC. When it finds that another computer is the PDC, it simply stops its' NetLogon service. The only way to fix this is to edit the registry at:

HKEY_LOCAL_MACHINE\Security

Choose Security / Permissions and insure that Administrators have Full Control on this key and all sub-keys. Navigate to:

HKEY_LOCAL_MACHINE\Security\Policy\PolSrvRo and change the <no name> value entry from 03000000 to 02000000 where the 3 indicates Primary and the 2 indicates Backup.

Shutdown and restart Windows NT on the old PDC. It will restart as a BDC which you can then promote.