var list = db.Employees.Where(p => p.empid == c.Employeeid).FirstOrDefault();
This is your problem .. i believe you are trying to read your record before inserting it.
try to pass the 'cmp' instead of the 'list' like this: (if cmp is not the right object type then use Employee)
public void insertEmployeeData(cmp c)
{
CompanyDataContext db = new CompanyDataContext();
db.Employees.InsertOnSubmit(c);
db.SubmitChanges();
}
set the empid for the 'cmp'(or Employee) object if it is not set automatically.