The problem starts from this method
private void store_detailOperation()
You opened a connection here and performed a database operation and leaving the connection open in here
DataSet ds = new DataSet();
SqlDataAdapter da = new SqlDataAdapter("select * from store_detail where prdid='" + mtbprdid.Text + "'", ConnectionString.cnn);
int s = (da.Fill(ds));
so after this invoking 'store_detailUpdation()'
inside these method you are trying to open the connection again which is already opened for above select statement. and It's very obvious from the error message.
it's a common and best practice to add a Connection state checking before you try to open database connection again.
i.e add the conneciton state checking inside the 'store_detailUpdation,store_detailinsertion
methods as shown below
public void store_detailUpdation()
if(ConnectionString.cnn.State == ConnectionState.Open)