Problem scenario
Atlassian JIRA is not working. The web UI does not have a login page. On the back-end in the logs, you see an error about not finding a suitable driver. (You may get an error about a port number instead. This problem may require a similar solution to the one proposed below for an error about a suitable driver.) You know you have the ojdbc.jar file (or variation thereof) in the correct place. What should you do?
Solution
Verify the dbconfig.xml file is configured properly. The individual stanzas should each be verified for accuracy. If the database hostname was entered incorrectly (and this will not necessarily be obvious), the error you get could misleadingly refer to a database driver. The real problem could be a typo in the hostname for the database. An incorrect database server or network disconnect between the JIRA server and the database server could be the real problem.
If the ojdbc.jar file became corrupt, the error may not have been misleading after all. You may actually need a new .jar file from Oracle's website. For more information, you may want to see this link.