ayc,
Go to the DB Browser view, edit the profile you are using, verify that the details are correct and unchanged.
Then go to the Drivers Preferences and edit your Oracle driver (the driver referenced by your profile), is it still pointing to the right driver JAR file (maybe you moved it)?. To be sure you can go to the Driver Libraries tab, remove the jar file listed and readd it.
Does it work?
If not, I’m going to ask you to follow the instructions here Posting Guidelines which will help us resolve your issue asap.
Best,
Brian.