ORA:12505: TNS:listener does not currently know of SID given in connect descriptor

suddenly got error on one computer

ORA:12505: TNS:listener does not currently know of SID given in connect descriptor

for test : after  installing on new computer with configuration ,everything works

so what happened to old computer ?
techpAsked:
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QlemoBatchelor, Developer and EE Topic AdvisorCommented:
You would need the (exact) connection string of the application, including a valid username and password. But for the connection test user and pwd do not matter, the hoststring or connection string is the alias defined in TNSNames.ora.
A complete connection string would look like this:
   user1/pwd1@db1
if db1 is defined in TNSNames.ora like
   db1 = sever1/instance1
(probably the string after = is much longer and contains a complete connection descriptor).

Do you know what the application uses for connecting?
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slightwv (䄆 Netminder) Commented:
The listener on the other machine isn't listening for connections for the instance you provided.

From a cmd prompt on the database server:
lsnrctl status

Look for the instance you want.  If it isn't there then it is one of two things:
The instance is down
or
Hasn't registered itself with that listener.

As for why it happened all of a sudden:  What changed?
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QlemoBatchelor, Developer and EE Topic AdvisorCommented:
Are talking about servers or clients by "old" and "new"?
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techpAuthor Commented:
client
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QlemoBatchelor, Developer and EE Topic AdvisorCommented:
Check the local tnsnames.ora, if used against the "new" one.
How is the connection info supplied to the application?
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techpAuthor Commented:
i replaced tnsnames.ora from new computer . same error
 (system is developer 2000 client)
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QlemoBatchelor, Developer and EE Topic AdvisorCommented:
Is sqlplus available for testing the connection? If so, try it.
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techpAuthor Commented:
sql plus 3.3 available...what is username password hoststring ? and what to test ?
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techpAuthor Commented:
thank you olemo
 i changed database instance from sql net and it became ok
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