Only way I have salvaged something like this...
find 2 other crop marks from another finished/cut print...
Take your unfinished print and carefully measure out what would be the bounding box for your print and mark it out with stabilo or marker to find missing two corners...
Then look at how the front crop marks are positioned...then crop and stick your new crop marks mirrored off the trailing edge where you think they should go to replace your missing ones...
Make sure you measure carefully from another print, watch for bleed in your file and do your math right...
I saved a fairly long print a couple weeks ago like this...it stopped for no reason just before printing rear crop marks...I spooled out more material past my print, then printed two new crop marks and cut it off leaving enough to cut it later, then on the table I did some math and drawing with ruler, trimmed around new crop marks, peeled them off and flipped them around and stuck them where my measurements thought they should be...worked out fine but I did have 1/4in bleed to play with...