I'm trying to diagnose an issue at a customer site. They are receiving the following error:

A network-related or instance-specific error occurred while establishing a connection to SQL Server

I've seen this a few times, but only during the initial setup - it's often caused by one of the following:

• The database server is turned off
• The network connection between the database server and the application is closed or somehow blocked (e.g. a firewall)
• The SQL Server instance is not set up to receive remote connections from the application server (e.g. TCP is turned off, remote connections are disabled, or the "SQL Server Browser" service is stopped/disabled)

However, if I assume that no configuration changes have been made, I'm trying to postulate on what the reason might be for getting this error at a random point after the initial setup. My initial thought is:

• SQL Server machine has run out of resources (e.g. RAM) and is unable to accept new requests from the application server

Is this a valid theory? What other possible causes are there of this error that are not related to the initial setup of the server / application connection? Or is it simply impossible that this error could occur without a configuration change having been made (either on the SQL Server side, application side, or somewhere in-between (network))?

NOTE: I believe this question differs from the plethora of questions related to this error message because the application and server have been talking to each other quite happily until now (most, if not all, other questions seem to relate to initial setup).