Can any workstation or server connect to the server? If so, then the server is operating properly and you should diagnose the client.

Is the database running? If not, start the database and retest the connection.

Is the listener running? If not, start the listener and retest the connection.

Can a loopback test be performed on the server? If so, you should diagnose the client. If not, the problem lies either in the SQL*Net configuration or in the network protocol stack on the server host.

If you are using Oracle Names, is the Names server running and reachable? If not, start the Names server or correct any connectivity problem.