Anyway to find out who shutdown oracle database

Your database is looping as it attempt to do a recovery and bring undo segments back online to recover your database. Since you are not a dba, I would immediately get oracle support on the line and tell them that you need major assistance and that you are not a dba. They will very likely have you startup the database and disable the recovery manager that is having smon loop trying to clean up undo segments. Having had to do undo segment cleanup in the past, it is best done with oracle supports help. Much of the steps will require some events and underscore parameters that disable/enable hidden oracle mojo. I would not recommend attempting anything you find on your own or is posted without understanding the implication and possibly how to recover/back out of your attempts. The object is to recover your database and not make it worse. Get oracle support on the phone asap.

Try the following:

select s.username, s.sid, p.spid 
  from v$session s join v$process p on addr=paddr
where s.username = 'USER2';

Using the spid value from the above query, log on to the server and issue the following command from a DOS command prompt:

orakill YOURSID spid

YOURSID is the SID of the database instance you're dealing with.

orakill often works when the GUI tools fail to kill processes. Here is a nice overview of orakill. Note reason #1 for using orakill - it might explain why you can't use your GUI tool:

The alter system statement will not clear any locks that exist. Instead, the session will remain connected until it times out, then the session is killed and the locks are released. The orakill command will kill the thread and the locks instantly.

Update:

You can also try:

select sid, serial#, program from v$session;
alter system kill session '<SID>,<SERIAL#>' immediate;

although I wouldn't hold out a lot of hope...