Go to your alert log file for this instance and first confirm the cause of the problem. Then log into the server that runs this instance, then go to the dos prompe and follow the steps


setenv ORACLE_SID=ur_instance_name


C> svrmgrl
SVRMGR> connect sys/sys_password as sysdba
SVRMGR> startup mount
SVRMGR> recover database;
SVRMGR> alter database open;
SVRMGR> exit;

This would mount the database, recover the database if it was in the archivelog mode, then open the database for the regular use.

sam