Server Error in '/' Application.
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: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) 

Тази грешка WEB сървъра я подава, когато не е добре конфигуриран Connection string и приложението не може  да се свърже с SQL сървъра. Моля, проверете дали е от вида:
Server=mssql.asphostbg.net;Database=databaseName;Uid=userName;Password=password;