This is how i resume my prints that don't make it all the way. this instructable only works if :
A: the print has failed cleanly meaning still stuck on print bed and no spaghetti monster.
B: your printer has not skipped steps*
C: you have the g-code (you don't want to re-slice.
* you can fix skipped steps in x and y if you don't have autobed leveling. and you can fix z if you home up (some mendel 90 and Delta printers)
Step 1: Stay clear!
move your nozzle clear of the print. get it some z height to to make sure you not ramming your print of the bed.
remove any blobs that may mess up your print at resume.
Step 2: Fix you extruder
if your extruder is clogged fix it now. keep a piece of paper under to protect you print. Note: move your axels before working on the extruder this will engage the motors holding it in place. yes it is possible to re home your x and y but if you can restart without loosing steps why not?
Step 3: Find z height
carefully find the height your print failed at. Do this the same way you level your bed= feeling it, I use a paper. try to find a good flat spot to do this on. Note that the layer may not be completed, measure the highest layer.
use a M114 and note down your z. mine is 122.10
Step 4: Fix g code
find the g code you where printing (you dont want to re-slice). make a copy and open in a text editor.
use crl+f to find the layer your print failed on (crl+f= z122).
in step 3 i found my z to be 122.1 in my g code one layer had z 121.9 and the next one 122.4
you want to delete all text before the row that say "NEW LAYER" follow by (in my case) 122.4
then save your file.
Step 5: Upload g code
upload your new file and start heating up your printer!
Step 6: Prime your extruder
prime it. but remember to do a M92 E0 after or your extruder will retract at print start. If you need to zero your x and y now.
Step 7: Hit print!
Do a little dance and hit print!
Step 8: Enjoy
If all went well your print will make it this time.
Now post a picture of your failed and resumed print in the comments :)