Have a look in the subfolder hts-cache of your mirror.
If old.zip is much bigger than new.zip, there may be a solution to recover
most of the vanished files.
Make a copy of the whole mirror and delete new.* rename old.zip, old.txt and
old.lst to new.zip...
Launch httrack with this modified mirror, tick do not purge old files in the
options, launch the mirror (and interrupt it) to recover the cached files (it
may work).