if you put the CD in your burner, and run Nero, click the medium info button, and (assuming it is a multisession CD) select the last session, if it is not closed there will be a couple of buttons in the window that let you close the session / finalize the CD.
i'm not sure however what would happen if this disc wasn't burned with Nero in the first place.
you can use Nero InfoTool -> Disc tab to see what program burned the CD.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the land of Mordor, where the Shadows lie
M.C. Escher - Reptilien