Sorry, but you can't reformat a CD-R or DVD-R, only a CD-RW or DVD-RW. Also, you probably closed off the disc to further change. Once you close off a disc you can no longer make any changes to it or burn any new files to it, even if there is free space.
Last time I burned a disc with Nero Start Smart but I can't reformat the disc. When I tried to reformat it at My Computer, it says the disk is write protected or something like that. Do I need to use other programs to reformat the disk.