The NBI file would make things easier of course, but you can recover the data manually. Just double-click on any .NCO file and even without having Nero installed on the system, a windows Explorer window will open and allow you to manually restore the files. If the content was Password Protected, a window will pop up asking for the password, and then the windows explorer window will apear to allow you to manually restore the files.
As for the missing NBI file, I hope the last disc is not missing as you may loose some data if it's not just a case of our software not writing the NBI file to the last disc.
You could contact the company that did the back for you, and if they have not erased the system, they could go to the below directory and locate the missing NBI file as our Nero BackItUP software always writes the NBI file to the last disc, and it makes a copy on the C: drive in the below location as a backup, just in case.
C:\Documents and Settings\<Your Computers Name>\Application Data\Ahead\NeroBackItUp\Info Files
You need to grab the NBI file as well as the NB.txt file. You'll need need to do the followin steps...
1) Place the last disc in your Recorder
2) Copy the contents of the last disc do your Hard Drive by manually draging and dropping to a folder using Windows Explorer
3) Place the NBI and NB.txt file in the same folder
4) Now using Nero Express or Nero Burning ROM, burn the contents of this folder that contains the backup data and the NBI and NB.txt file to a CD or DVD.
5) Now try and restore selecting the NBI file from the disc you just created.