ORA-01012: not logged on

You have either not logged in or something closed your connection.

What application/program/??? is generating that error?

Have you checked the description for that error:
http://ora-01012.ora-code.com/

I am trying to login on sqlplus "/as sysdba"

Is the databsae started and 'open'?

from the sqlplus prompt type: startup

If it says it is already started try: alter database open;

I've seen a few posts on other sites that indicate this can also be caused by exceeding the number fo processes.

check the processes spfile parameter.  You might need to increase the value:
http://docs.oracle.com/cd/E11882_01/server.112/e25513/initparams198.htm

I tried but getting error.

SQL*Plus: Release 9.2.0.8.0 - Production on Wed Mar 28 10:48:05 2012

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Connected.
SQL> startup
ORA-01081: cannot start already-running ORACLE - shut it down first
SQL> shutdown
ORA-24324: service handle not initialized
ORA-24323: value not allowed
ORA-27140: attach to post/wait facility failed
SQL> shutdown abort
ORA-01031: insufficient privilegesseelct * from v%


and after kiiling some of the processes
now I am stucked

SQL> shutdown abort
ORACLE instance shut down.
SQL> startup
ORA-00445: background process "DBW0" did not start after 120 seconds

What was the final solution to the problem?

I doubt the accepted answer actually solved the question asked.

Please return to this question and respond to my question.