I have this problem on the web:
I can't run anymore a simple form which needs to access to the Database but I can run a simple form which doesn't need any access to the Database. The first form worked well.
Now with the first form it doesn't appear the logging session for the Database and it doesn't appear anything anymore.
On the computer where the web/app server is hosted, open a command prompt and issue the command TNSPING <SID> where SID is the name of your Oracle database. For example, "TNSPING ORCL". If this errors out, then you need to check Oracle Net components on the client (basically files tnsnames.ora and sqlnet.ora) to make sure they are correctly pointing to the Oracle Listener on your database server.
If it used to work, but now it does not, the problem could be the host address was changed. The host address has three elements: protocol, hostname (or IP address), and port number. Look at the file listener.ora on the server to see what address it is listening on, and then make sure your web/app server's copy of tnsnames.ora is set to use that exact address. For example, this problem can occur when DHCP is used and the server is issued a new IP address.