[henry]

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.