One of my customer accidently set the "max server memory" of SQL Server to 0, and then the server can't start after that. If we try to start SQL Server from SQL Server Configuration Manager, it failed to start (esp. when you have several active databases), and in the SQL Server error log file, we could find a error message like "... a internal query can't be executed due to inefficient memory...". Or, sometimes, SQL Server can be started, but we can't create a connection to it with the following error message:

In this case, we could add -f to SQL Server startup parameters which allows SQL Server to start with minimum configurations therefore uses least memory. After SQL Server starts, we could connect to SQL Server using SQL Server Management Studio and update the "max server memory" to an appropriate larger value.

A bit more details: if we set the "max server memory" to 0 via SSMS-> Server's Properties -> Memory, SSMS will automatically adjust the value to minimum account of memory for option "max server memory"; for SQL Server 2005-2008 R2, it's16MB, and for SQL 2012, it's 64MB for 32-bit, and 128MB for 64-bit server box; if we set the "max server memory" to 0 via sp_configure stored procedure directly, SQL Server will automatically use default "max server memory" value which is 2147483647MB.

References:
Server memory server configuration options: http://msdn.microsoft.com/en-us/library/ms178067(v=sql.110).aspx.
How to: Start an instance of SQL Server: http://msdn.microsoft.com/en-us/library/ms180965(v=sql.105).aspx

Hope this helps!