There can be so many issues but if you are using oracle 10g , uninstall oracle 10g and also remove the value from registry and install oracle 11g. But if you are using oracle 11g , first go and check on registry if it is pointing to right home. Sometimes there can be more than one home because you install sql developer again and again . In that case either you can remove unnecessary home registry value or you can add tns and sql.net file to all of those home, that might resolve the issue. I resolved mine in that way.