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.
If a support structure has collapsed or otherwise broken, while the print head is out of the way I can quickly shore up or anchor a trouble spot. Shine UV on it and it will cure hard. (the Bondic dispenser comes with a small UV LED light, but I use a higher power handheld unit I bought online.)
It is very important not to get any of the resin on the print layer where the print head could get contaminated. Only apply to areas that have already been printed.
On more than one long print job, I have salvaged a weak or broken support structure by applying a bit of Bondic in the right place. Here is a recent example of providing just enough stability that the extruder eventually re-established the support.
|In the UV, the resin glows bluish. Here you can see the mid-air loose printed filament anchored to the neighboring pillar.
|Here you can see that printing has picked up again, successfully! Since this is just support material, it doesn't matter much that things went wrong in the middle - it only really matters how it ends.
|Side view shows the earlier break, and the subsequent re-established support above it.