Seems database is not up. It might be due to restarting machine and the instance is not set to autostart and it so not started manually after starting from services Screen.

Just goto Command prompt

Set Oracle SID C:>set oracle_sid=ORCL

Now run Net start command. C:>net start oracleserviceORCL

Please try this once..

Maybe your database is not up. If the machine was restarted and the instance is not set to autostart (and it was not started manually), you may need to start the service yourself.

If you have access to the Services screen, you can do it from there; or, you can do it from the command line.

Go to Command prompt and enter the following commands:

set oracle_sid=ORCL

net start oracleserviceORCL

The first sets the Oracle SID; the second actually starts the service.