Since you are encountering this problem after a server reboot, check to see that the instance has started and the database is open. On the Windows platform, an Oracle database can be started automatically on server startup via the service. The most common cause of this problem is when the STARTMODE of the service is set to manual. When the service for the database is created with the oradim utility, you have the option to specify a STARTMODE as either AUTO or MANUAL. It's possible that the service was created with the STARTMODE as MANUAL, in which case your database would not start up automatically when the server reboots or starts up. To change this, you can recreate the service with the oradim utility (after shutting down your database). You can use the command:
oradim –edit –sidIt is also possible that the service is starting up but not the database. In this case, it is possible that the STARTTYPE was set to SRVC rather than INST. This would start the Windows service but not the database itself. This, too, can be modified with the oradim utility.
This was first published in February 2006