An object reference is required for the non-static field, method, or property

 public static  string getData(string _mobileno)
        {
            if (_mobileno == null || _mobileno.Length == 0)
            {
                return String.Empty;
            }
            else
            {
                OleDbConnection conn = null;
                string cnstring = ConfigurationSettings.AppSettings["ConnectionString"];
                conn = new OleDbConnection(cnstring);
                string _strQry = "Select distinct VISITOR_NAME from VISITOR_REG where MOBILE_NO=" + _mobileno+"";
                conn.Open();
                OleDbCommand cmd = new OleDbCommand(_strQry, conn);
                cmd.CommandType = CommandType.Text;
                cmd.Parameters.AddWithValue("MOBILE_NO", _mobileno);
                string name = Convert.ToString(cmd.ExecuteScalar());
                setvalue(_mobileno);
                return name;
                

}