I've made an error!
 

I've written files on a DVD (no RW) with Nero 6 a week ago. Today, I've made a mistake: when I opened the project in Nero, I've deleted all the file on the project and then I've written on the DVD. Now I can't view the file I saved on the DVD a week ago. Can I recall this file or I've lost them forever?

You can indeed open a previously written session in Nero, pop the disk in the writer and select 'continue multi-session disc'.
You can now select the session the data you lost was written on, be careful or you'll lose your current data as well.

Personally I never use multi-session on DVDs, I wait until I can burn a full disk.
As a rule 1gb is the minimum a DVD multi-session can burn.

Problem solved! Thank you very much for your advices! ;)