OK let me try
There is a process, called listener out of the database. It is responcible for getting u connected to the database. When the listener does not knows of about the database because it is down or still have not registered to it, the listener cannot connect u
On the other hand, when logged at the machine where the DB is installed, and u use SQL Plus to
conn / as sysdba
the listener is not used for connecting u. SQL PLus by itself gets to the oracle home directory and does what's necessary
However, if U use
conn sys/password@string as sysdba
the @string part points u to the listener and since listener do not knows of your database, the conn fails
iSQL plus allways goes trough the listener so no chance to pass trough SQL plus
I hope I have not missed many things and described it in a way u can understand

Regards