The easiest solution could be to ask a colleague for assistance and update the tnsnames.ora file with the required connection details. Sometimes, this file might get overwritten or emptied after certain modifications. In such cases, you can simply copy and paste the correct connection details back into the file, and it should work. After this, you should be able to see all the connections you added in the tnsnames.ora file listed under the 'TNS Network Alias'.