I'm not sure how you verified the username and password were correct, but, what I usually recommend first is to make sure that your login string is correct outside of Data Protector
On the Oracle server, run
sqlplus /nolog
connect username/password@service as sysdba
Of course, you would use the Username and password that you are trying to connect with and 'service' can be found in the 'tnsnames.ora' file, although, more often than not, it is the name of the instance
If this doesn't connect, then the problem is on the Oracle side
If it does connect, I suggest that you redo the Integration, as the Configuration file may have become corrupt. This can be done either from the GUI in the Backup -> Oracle context, right-clicking on the Oracle server, and pick Configure, or, from the command prompt on the Oracle server
You didn't specify what version of DP, so I can't be more specific, but, the procedure for either method is in the Oracle Integration Guide