When it happens again, you could try to connect to the SQL instance using a "Dedicated Admin Conection (DAC)". If you even can't connect to the Windows server when the problem occurs, you'll need to enable the "remote admin connection" setting with sp_configure. This setting will allow to you to open a DAC connection from another machine.

You can only open one single DAC connection at a time, so when you're using SSMS for this you don't use the Object Explorer for this. Open the DAC with a new Query window.

Kendra Little has made a blog about the DAC connection: http://www.brentozar.com/archive/2011/08/dedicated-admin-connection-why-want-when-need-how-tell-whos-using/