Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. The statement has been terminated

I have many users on my web site (20000-60000 per day), which is a download site for mobile files. I have remote access to my server (windows server 2008-R2).
I've received "Server is unavailable" errors before, but I am now seeing a connection timeout error.

I'm not familiar with this - why does it occur and how can I fix it?

The full error is below:

Server Error in '/' Application. Timeout expired. The timeout period
elapsed prior to completion of the operation or the server is not
responding. The statement has been terminated. Description: An
unhandled exception occurred during the execution of the current web
request. Please review the stack trace for more information about the
error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: Timeout
expired. The timeout period elapsed prior to completion of the
operation or the server is not responding. The statement has been
terminated.

Source Error:

An unhandled exception was generated during the execution of the
current web request. Information regarding the origin and location of
the exception can be identified using the exception stack trace below.

Stack Trace:

[SqlException (0x80131904): Timeout expired. The timeout period
elapsed prior to completion of the operation or the server is not
responding. The statement has been terminated.]

System.Data.SqlClient.SqlConnection.OnError(SqlException exception,
Boolean breakConnection) +404

System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning() +412

System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior,
SqlCommand cmdHandler, SqlDataReader dataStream,
BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject
stateObj) +1363

System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds,
RunBehavior runBehavior, String resetOptionsString) +6387741

System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior
cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean
async) +63

Looks like you have a query that is taking longer than it should.
From your stack trace and your code you should be able to determine exactly what query that is.

This type of timeout can have three causes;

• There's a deadlock somewhere

• The database's statistics and/or query plan cache are incorrect

• The query is too complex and needs to be tuned

A deadlock can be difficult to fix, but it's easy to determine whether that is the case. Connect to your database with Sql Server Management Studio. In the left pane right-click on the server node and select Activity Monitor. Take a look at the running processes.
Normally most will be idle or running. When the problem occurs you can identify any blocked process by the process state. If you right-click on the process and select details it'll show you the last query executed by the process.

The second issue will cause the database to use a sub-optimal query plan. It can be resolved by clearing the statistics:

exec sp_updatestats

If that doesn't work you could also try

dbcc freeproccache

You should not do this when your server is under heavy load because it will temporarily incur a big performace hit as all stored procs and queries are recompiled when first executed.
However, since you state the issue occurs sometimes, and the stack trace indicates your application is starting up, I think you're running a query that is only run on occasionally. You may be better off by forcing SQL Server not to reuse a previous query plan. See this answer for details on how to do that.

I've already touched on the third issue, but you can easily determine whether the query needs tuning by executing the query manually, for example using Sql Server Management Studio. If the query takes too long to complete, even after resetting the statistics you'll probably need to tune it. For help with that, you should post the exact query in a new question.

Stack Overflow Q#8602395, score: 467