I got that error when I had accidentally moved my tnsnames.ora file so SQL Devloper couldn't find it. See if your tnsnames.ora is where SQL Developer expects it to be.
Do you have an oracle client installed? The error implies you are using a connection method which requires OCI, which in turn requires ocijdbc. The JDBC version in SQLDeveloper and the client version must match (e.g. 220.127.116.11).
If you don't have a client installed, make sure you have Use OCI/Thick Driver unchecked in Tools|Preferences|Databases|Advanced.
If that doesn't work, you will need to post details of how your connection is defined.
Thanks for the replies. I got that error when using TNS for the connection type. When I switched to "Adavanced" it worked.