error while loginto my web application

i meet this error while open web page which connected to sql database

Login failed for user 'IIS APPPOOL\test'.

Description:An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: Login failed for user 'IIS APPPOOL\test'.

Source Error:

Line 20:         using (SqlCommand com = new SqlCommand("login", cn))
Line 21:         {Line 22: cn.Open(); 

[SqlException (0x80131904): Login failed for user 'IIS APPPOOL\test'.]