Lost all SQL Admin logins -- this command comes to my rescue.

PsExec -s -i "C:\Program Files (x86)\100\Tools\Binn\VSShell\Common7\IDE\Ssms.exe"

This will use Network Service to Login and you can add new login one you logged in through,

​Note: SSMS.exe path may differ, please check on your drive for SQL Installed Binn.​