Big job failed. want to resume

Depends on how confident you are with your print settings. I've done this exactly once, and the result was not perfect. Without removing the part from the build plate or the build plate from the machine, I used calipers to measure the height of my failed print+build plate. I subtracted out the thickness of the build plate, and then subtracted the first layer height (which is squished into the build plate and thus slightly smaller than subsequent layers). I took this new height and divided it by my layer height to get the number of layers.

However, if your height is too small, when you resume the print, you will be air-printing, which is BAD. If your height is too tall, your first layer or two will be squished a bit (the deflection will be absorbed by the extruder and print bed, so not too bad). So I added a fudge factor of .2mm to make the height a little taller than I thought it should be.

There are two routes to go from here:
1. I then went into my gcode and trimmed out those layers, and sent in the new print. When I did this, the top of the print hit almost hit some of my printer internals because the printer still homes like usual. I was very lucky in this regard, it could have been very bad. The print then resumed on top of the previous print but it was not perfect. There was about .5mm of offset, I guess from a combination of slight movement from measuring with calipers and general backlash of the extruder. The first new layer DID majorly squish into the existing print and I could see the force pushing the extruder and bed a tiny amount, but again, I think this is preferable to air-printing. The final part was about .3-.4mm shorter than it should have been (even accounting for normal shrinkage and whatnot). Not perfect, but better than total failure 16 hours into a 21 hour print.

2. Here's what I should have done and what I would do if I were to start over. Take the height of your part that printed before failure, go into your model of the part and literally chop off the bottom 2/3rds of it, re-export the file as a new STL, and print THAT. Then glue them together. The result should be much nicer and it is a LOT easier, and there are fewer ways to fuck up.

Good luck!

Edit: I should mention that this was NOT with a SD or Repetier-Host, so YMMV.