Occasionally, SCCM will have a hiccup, and ‘Blocked’ machines may lose visibility in the Devices – All Systems collection. So much fun.
For example, when a machine is blocked, normally you can just see the blocked status in the column header Blocked—it will say Yes or No—and you can easily change that status from the right-click menu. But, for some reason, SCCM will decide not to show the machine. Well, then…how do you unblock it??? The answer is to query the database, find the missing machine, and to Update the record to an unblock status.
On the CM Database, run this query
SELECT * FROM ClientKeyData WHERE IsRevoked = 1
That will list all of the clients that have been blocked.
Now, you could just unblock all of them using this SQL command
UPDATE ClientKeyData set IsRevoked = 0 WHERE IsRevoked = 1
Or, be selective, using these commands, assuming you can find the exact SMSID
SELECT * FROM ClientKeyData WHERE SMSID in (‘GUID:E0C8BBA7-142D-1628-A147-328AD3DA39CA’)
UPDATE ClientKeyData set isrevoked = 0 WHERE IsRevoked = 1