Las paginas que utilizan base de datos me mandan los errores (Win32Exception (0x80004005)) y SqlException (0x80131904)

Es el primer sitio web que publico y estoy teniendo este error en las paginas que tienen alguna conexión o necesitan de la base de datos para mostrar algo.

Error del servidor en la aplicación '/'. La ruta de red no se encontró Descripción: se produjo una excepción no controlada durante la ejecución de la solicitud web actual. Revise el seguimiento de la pila para obtener más información sobre el error y dónde se originó en el código.

Detalles de la excepción: System.ComponentModel.Win32Exception: No se encontró la ruta de red

Error de origen:

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.

Seguimiento de pila:

[Win32Exception (0x80004005): The network path was not found]

[SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)] System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions, SessionData reconnectSessionData, DbConnectionPool pool, String accessToken, Boolean applyTransientFaultHandling, SqlAuthenticationProviderManager sqlAuthProviderManager) +907 System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection, DbConnectionOptions userOptions) +334 System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnectionPool pool, DbConnection owningObject, DbConnectionOptions options, DbConnectionPoolKey poolKey, DbConnectionOptions userOptions) +38 System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection) +699 System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection) +89 System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions userOptions, DbConnectionInternal& connection) +426 System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection) +78 System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection) +191 System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 retry, DbConnectionOptions userOptions) +154 System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 retry, DbConnectionOptions userOptions) +21 System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource1 retry) +90 System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource1 retry) +443 System.Data.SqlClient.SqlConnection.Open() +96 SanRiver_v1._1.Alquiler.ConsultarIMG() in C:\Users\yorda\Desktop\Trabajo\SanRiver_v1.1\Alquiler.aspx.cs:43 SanRiver_v1._1.Alquiler.Page_Load(Object sender, EventArgs e) in C:\Users\yorda\Desktop\Trabajo\SanRiver_v1.1\Alquiler.aspx.cs:97 System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +51 System.Web.UI.Control.OnLoad(EventArgs e) +95 System.Web.UI.Control.LoadRecursive() +59 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +678

Según yo creo que es el ConnectionString pero ya lo intente cambiar varias veces y sigo sin arreglarlo.