Check the service password. Maybe it has expired or changed.

Can you even connect using the management studio from b(dmz) to a(private). That way you can see if your ports are even squared away before you start to play around...

So, corrct me if I am wrong, but you have only a c drive for this machine(s) and this is where the database, logs,windows...all live? If this is the...

Might have a file level permission problem. Make sure the account has appropriate file level permissions and that it has been granted appropriate rights to run the service.