C# sql exception, help needed

check these things
1. Make sure your database engine is configured to accept remote connections(I you are using centralized database)
•Start > All Programs > SQL Server 2005 > Configuration Tools > SQL Server Surface Area Configuration
•Click on Surface Area Configuration for Services and Connections
•Select the instance that is having a problem > Database Engine > Remote Connections
•Enable local and remote connections
•Restart instance

2. Check the SQL Server service account
•If you are not using a domain account as a service account (for example if you are using NETWORK SERVICE), you may want to switch this first before proceeding

3.If you are using a named SQL Server instance, make sure you are using that instance name in your connection strings in your ASweb P.NET application
•Usually the format needed to specify the database server is machinename\instancename
•Check your connection string as well

From link[^]:

Basically, when you failed to connect to your SQL Server, the issue could be:
1) Network issue,
2) SQL Server configuration issue.
3) Firewall issue,
4) Client driver issue,
5) Application configuration issue.
6) Authentication and logon issue.

Based on the error message you didn't allow the remote connection to that machine so change the options in settings.

Check these
Steps to troubleshoot SQL connectivity issues[^]
SQL Server 2005 Connectivity Issue Troubleshoot - Part I[^]
Troubleshoot Connectivity Issue in SQL Server 2005 - Part II[^]
Troubleshoot Connectivity Issue in SQL Server 2005 - Part III[^]

BTW check this bunch also for future errors

make sure Your Server Name

run this

SELECT @@SERVERNAME

copy paste the server Name

SqlConnection con = new SqlConnection("Data Source=./SQLEXPRESS;AttachDbFilename=C:/... Security=True;Connect Timeout=30;User Instance=True; ");