Thank you, aarroyob, for answering.

I've just checked my TNSNAMES.ORA and it was included:

MY_SERVICE.WORLD =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(Host = 127.0.0.1)(Port = 1521))
(CONNECT_DATA = (SID = MY_SID))
)

And I also checked my SQLNET.ORA and it includes:

names.default_domain = world

The name of my SID is MY_SID and the service is MY_SERVICE. I'm able to connect it via SQL*Plus, but it's impossible to connect it as a service.