How to resume package building in debian?

To continue a build that was interrupted for some reason, you can call the appropriate targets of debian/rules directly:

debian/rules build

will compile the sources, then

fakeroot debian/rules binary

will run the installation and prepare the packages.

I have been using the -nc switch for this. According to the man page:

-nc, --no-pre-clean
    Do not clean the source tree before building (long option since
    dpkg 1.18.8). Implies -b if nothing else has been selected among
    -F, -g, -G, -B, -A or -S. Implies -d with -S (since dpkg 1.18.0).