Option 1:
@acc_no
is not an OUTPUT parameter, make it an OUTPUT parameter, and do this in your SP.
SET @acc_no = SCOPE_IDENTITY()
Also tweak your C# code to make it an output parameter.
Option 2:
Instead of a return statement, use
SELECT SCOPE_IDENTITY()
in your Stored Procedure and use
cmd.ExecuteScalar()
in your C# code.