I was recently reading though the 3D Printing group on facebook when one of the members, Ken Jones, posted an amazing trick to save a print when the support structure falls off. Just use putty or clay to hold it in place.
I pause the print... remove the fallen supports... then put clay in across the horizontal gap, trying to make the top level of the clay as flat as possible and wide enough to serve as the new surface... I use the existing structure to anchor the clay on as well. It takes a little trial and error but eventually (when you get the clay slightly above the level of the print so the extruder comes in contact with just a tiny bit of the clay on the first pass) the supports start printing from where they left off on the fallen part. There are comments on each of the pictures. If this is too hard to follow, I can try to post a better explanation. Let me know.
It usually take a couple of passes but eventually the support fills back in saving the print. Neat trick, hopefully this saves some headaches for some one in the future.