Are you able to close the Session using Nero's Recorder>Medium Info?
Place the CD you attempted to create in your recorder, and launch Nero and go to Recorder>Choose Recorder.
If the session is not properly closed on the CD, you will get two buttons in the bottom left corner of the window that comes up. One will say 'Close Session' and the other 'Close Disc'. Use Close Disc if you do not plan on writing to the CD again, or Close session if you do.