SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified for MDF file

SQL Network Interfaces, error: 26 - Error Locating Server/Instance
Specified for MDF file

i have added database file(.mdf) in my application using visual studio
inbuilt functionality. database is in App_Data folder. it is running fine.
but when i publish it and upload it to server it gives this error.
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)
i visited similar situation but confused. i don't know which version and
edition of sql server is running at server. i upload project using
filezila .My connection string is as follow in we web.config
<add name="ConnectionString" connectionString="Data
Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated
Security=True;User Instance=True" providerName="System.Data.SqlClient"/>
please describe process step by step is possible.
Thanks in advance.