On the left is a support column. It looks solid, but it's really just a very thin zigzag ribbon, and the taller it is, the easier it tends to be to snap.
I discovered a tool that is really useful to salvage some broken support pieces. On a longer print, all you really need to do is put some support and anchoring back where there is none. If you provide some structure and stability, and the printer has enough printing left to do, then the support should slowly be able to pick up where it left off. If you can anchor things, or at least provide some foundation without interfering with the new layers the head wants to lay down, then you can probably salvage the print.
At first I tried to use various glues - but they never really worked. Either they are too hard to do a get-in-get-out application (because you need to dodge the print head), or they are too messy, or they are too hard to apply where you need, or they don't set quickly enough.
I eventually settled on a product called Bondic. It's a small dispenser of UV-curable resin. The resin is a clear syrupy fluid that sets hard when it's hit by UV. It hits all the bases for an emergency fix.