create table ApplicationMessage_vw (continueindicator number, messageid number);
create table SystemLockout_vw (recordcount number);

create or replace trigger NO_ACCESS
after logon
on database
declare
  l_count number;
begin
  select continueindicator
  into   l_count
  from   applicationmessage_vw
  where  messageid = 1;
  --
  if l_count = 2
  then
    raise_application_error(-20001, 'System is not available');
  end if;
  --
  select recordcount
  into   l_count
  from   systemlockout_vw;
  --
  if l_count > 0
  then
    raise_application_error(-20001, 'Lockout already there');
  end if;
end;
/

SQL> insert into SystemLockout_vw values(1);

1 row created.

SQL> commit;

Commit complete.

SQL> conn scott/tiger
ERROR:
ORA-04088: error during execution of trigger 'SYS.NO_ACCESS'
ORA-01403: no data found
ORA-06512: at line 4

Warning: You are no longer connected to ORACLE.

SQL> conn sys
Enter password: ***
Connected.
SQL>