ORA-01089 when opening the database

We are encountering ths problem:
DATABASE OPEN: error when opening database:

ORA-01089: immediate 
 shutdown in progress - no operations are permitted.

Your database is trying to shut down. When it is trying to shut down, no one is allowed to log in. To find out why your database is trying to shut down, you'll have to look in your database's alert log file for clues.

Sometimes, the database won't shut down no matter how long you wait. So you'll have to force it down and then restart it. The following commands in SQL*Plus should do the trick:

connect / as sysdba
shutdown abort
startup