1. You discover that the netlogon service is in a pause state
2. Event ID 2095 / 2103 is logged in the directory service event log
3. Inbound and outbound replication is disabled
You’re in a now in an USN-Rollback state!
(This situation will never happen in a single DC forest)
The most common reasons for this state is that an unsupported restore method has occurred.
– You clone a DC with ie. VMWare Converter
– You revert to a snapshot of a DC and start the DC in normal mode without setting the “Database restored from backup” registry key 
If your HW is set to do buffered disc writes and an unexpected power failure occur, may also put you in an USN-Rollback state
The way to get out of this state is to remove the bad DC out of your domain:
1. on the bad DC: dcpromo /forceremoval
2. on a healthy DC: Run a Metadata cleanup 
3. on a healthy DC: seize the FSMO if necessary 
4. rebuild the “bad” DC and promote it back
I’m living in a freezing country (bah!) and we use to say “pee in your pants to get warm (for about 5 seconds…)”.
I have seen some recommendation at some other blogs and forums that provides a “quick” solution to recover from a USN-Rollback state.
Here is a “pee in your pants” solution:
– Delete the “DSA Not Writable” registry key
– Enable replication with repadmin
– Fixed? Nah, not really!
Here is what Microsoft says about this in an extention to KB 875495  that for some reason is hidden for the general public:
“Deleting or manually changing the Dsa Not Writable registry entry value puts the rollback domain controller in a permanently unsupported state. Therefore, such changes are not supported. Specifically, modifying the value removes the quarantine behavior added by the USN rollback detection code. The Active Directory partitions on the rollback domain controller will be permanently inconsistent with direct and transitive replication partners in the same Active Directory forest.”
 DC’s and VM’s – Avoiding the Do-over
 Backup and Restore Considerations for Virtualized Domain Controllers
 Metadata cleanup
 Seizing the FSMO’s
 KB 875495 – How to detect and recover from a USN rollback