Registered Server list and groups disappeared from SQL Server 2012 Management Studio
The list you're talking about wouldn't be present in Object Explorer (unless you connected to all of them manually every time you started Management Studio). Your active connections will disappear from Object Explorer when you close down SSMS or reboot, so you can't persist them there (as convenient as that might be).

What you might be talking about, though, is Registered Servers, which you might have inadvertently closed before closing SSMS last (it should pop up again if it was active when you shut down SSMS or rebooted). If it's missing, no problem, you can restore this panel using View &gt; Registered Servers or through the key combination Ctrl+Alt+G.

If your servers still don't show up there, it's possible that you lost your registered server configuration due to a roaming profile, logging in as a different user, or messing with SqlStudio.bin.