Help!! Cant get to files on backup disc!!!

[RC159]

Nero 5.5.0.3 using plextor burner. Here's what happened. I was copying a file from my backup cd to the hard drive to burn to a blank cd. I forgot to take the backup cd out and copied the file to the backup cd and renamed the disk backup to directx_8. The disc was a multisession disk. When I went to copy the directx 8 file I selected no multisession. When I burnt the file and opened the tray and realized that I had my backup cd in instead of the blank disk I figured that everything thing would still be there, but know only the directx8 file is there and it renamed the disc to directx_8. This is not a cdrw disc, how could it do that? When I right click on the disc in explorer and choose properties, then volumes, it shows the sessions from the other files, but when I clck on them it says "volume could not be mounted" anybody know whats going on? Is there anyway of getting the files back? Theres some important stuff on the disc that I'd like to get back. Please help!