If you cannot login to the database, for instance due to ORA-00020 maximum number of processes exceeded, then chance exists that you could use the -prelim option.
Documented in note 121779.1 for sqlplus version 10.1 and later :
In some cases, no connections are allowed on the instance (in some ORA-20 situations for example).
As of 10.1.x, there is a new option with SQL*Plus to allow access to an instance to
sqlplus -prelim / as sysdba
Only sysdba connection is possible.
sqlplus -prelim system/manager SQL*Plus: Release 188.8.131.52.0 Production on Mon Jul 4 10:38:36 2011 Copyright (c) 1982, 2010, Oracle. All rights reserved. ERROR: ORA-24300: bad value for mode
And very little access is granted
SQL> select * from dual; select * from dual * ERROR at line 1: ORA-01012: not logged on Process ID: 0 Session ID: 0 Serial number: 0
shutdown abort and then restart your database, instead of rebooting your server where other instances may be running.
This is the ultimate chance before reboot. Before this, consider disconnecting / killing some user session to get a regular sqlplus / as sysdba