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You may provide a little bit more information, because there may be 1001 different reasons.<br>Any changes, updates etc. in the past before it stopped?<br>Any error messages?<br>All services started?
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dave_sky

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Check if your Database is up while its service is started properly.
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Also check the machines Event Viewer for any error associated with your program.
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any issues with the firewall blocking?
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If you try to create an ODBC connection to the server on the system where the application resides, what's the result?
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I have a VB.Net 2005 application that will not connect to a sql server 2005 database.<br>Has been working for several years with no problem...
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I have a VB.Net 2005 application that will not connect to a sql server 2005 database.
Has been working for several years with no problem...

I have a VB.Net 2005 application that will not connect to a sql server 2005 database

Visual Basic .NET (VB.NET) is an object-oriented programming language implemented on the .NET framework, but also supported on other platforms such as Mono and Silverlight. Microsoft launched VB.NET as the successor to the Visual Basic language. Though it is similar in syntax to  Visual Basic pre-2002, it is not the same technology,

Visual Basic.NET

SQL (Structured Query Language) is designed to be used in conjunction with relational database products as of a means of working with sets of data. SQL consists of data definition, data manipulation, and procedural elements. Its scope includes data insert, query, update and delete, schema creation and modification, and data access control.

Microsoft SQL Server is a suite of relational database management system (RDBMS) products providing multi-user database access functionality.SQL Server is available in multiple versions, typically identified by release year, and versions are subdivided into editions to distinguish between product functionality. Component services include integration (SSIS), reporting (SSRS), analysis (SSAS), data quality, master data, T-SQL and performance tuning.