Nhibernate - Transaction not connected, or was disconnected

We are running a WCF service with Nhibernate and all of a sudden the following exception starts getting thrown:

NHibernate.TransactionException, NHibernate.

The stack trace shows:

NHibernate.Transaction.AdoTransaction.CheckNotZombied()
NHibernate.Transaction.AdoTransaction.Rollback()

Googling seems to bring up heaps about session management issues, however our problem was that the database log file was full.