Sharma is right; Seems like your Oracle service is starting but the database is failing to initialize.
That is the problem with NT Services; it does not tell you when it fails to start the database. Check the Alert Log and it will show you why the database was not started.
Or if you start from the SvrMgr, it will show you the errors.
also make sure you set oracle_sid before attempting to connect from the Server Manager.
On the DOS prompt --
SET ORACLE_SID = your_sid_name
If the SID is SET and the related Oracle service (OracleServiceSid_Name) is running then you should be able to connect from Server MAnager.
I think in Oracle 7.3 (for NT) there are 2 services; can you check that both the services are running.
And most importantly check the end of the Alert Log file for the appropriate errors.