The problem is in the Query!
You've written
cmd = new SqlCommand("select Member_Name,Father_Name,Corresponding_address,Designation,ValidFrom,Validupto from EmployeeDetails where StaffNumber='" + lbstaffno.Text + "'", con);
con.Open();
da = new SqlDataAdapter("select Member_Name,Father_Name,Corresponding_address,Designation,ValidFrom,Validupto from EmployeeDetails where StaffNumber='" + lbstaffno.Text + "'", con);
The problem is that while stating an int value in coding, we never enclose them in quotes!
All you have to do is just remove the single quotes from your query and you're done!
Change your query in the following manner:
cmd = new SqlCommand("select Member_Name,Father_Name,Corresponding_address,Designation,ValidFrom,Validupto from EmployeeDetails where StaffNumber=" + lbstaffno.Text, con);
con.Open();
da = new SqlDataAdapter("select Member_Name,Father_Name,Corresponding_address,Designation,ValidFrom,Validupto from EmployeeDetails where StaffNumber=" + lbstaffno.Text, con);