***
check "control panel --> services-->", the oracle service you added (say: OracleServiceSID) is started.
***

If not try start the service from control panel.

go to command prompt (DOS prompt)

set ORACLE_SID=SID

C:\> svrmgrl

SVRMGR> connect internal

Note: Asumption is ur database is installed on the same machine where u r trying the connection. Else, you need the TNS stuff like that.