This really should be a comment to Brad Rippe's answer, but alas, not enough rep. That answer got me 90% of the way there. In my case, the installation and configuration of the databases put entries in the tnsnames.ora file for the databases I was running. First, I was able to connect to the database by setting the environment variables (Windows):
set ORACLE_SID=mydatabase
set ORACLE_HOME=C:\Oracle\product\11.2.0\dbhome_1
and then connecting using
sqlplus / as sysdba
Next, running the command from Brad Rippe's answer:
select value from v$parameter where name='service_names';
showed that the names didn't match exactly. The entries as created using Oracle's Database Configuration Assistant where originally:
MYDATABASE =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = mylaptop.mydomain.com)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = mydatabase.mydomain.com)
)
)
The service name from the query was just mydatabase rather than mydatabase.mydomain.com. I edited the tnsnames.ora file to just the base name without the domain portion so they looked like this:
MYDATABASE =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = mylaptop.mydomain.com)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = mydatabase)
)
)
I restarted the TNS Listener service (I often use lsnrctl stop and lsnrctl start from an administrator command window [or Windows Powershell] instead of the Services control panel, but both work.) After that, I was able to connect.