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To add to Assim's solution, you want to check your SQLAgent and stop and disable all of the jobs. I've had to go in and do this for DBAs who were killing processes only to have another one spin up right behind it. Don't forget to reenable the jobs!
First thing never set Max Number of Concurrent Connections on server properties page to 1, as this option can even stop administrators to login but still in any case Dedicated Administrative Connection can have access. But if you have changed Max Number of Concurrent Connections value to 1 and now unable to login 1-Restart services using SQL Server ...
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