What causes a nozzle to clog? In my case, anytime "something happens" and plastic doesn't get pushed through a heated nozzle for an hour or more, I have clog issues. An unlevel platform, dirty drive gear, shrinkage and "upcurl" of a big part can cause the nozzle to quit extruding. If I "walk away" from a 14 hour build, Murphy's Law increases the chances of "no extrusion but the heat's on."
First, open a ticket with MakerBot support. They are excellent and will help you troubleshoot the problem. If the result is "clean the abs plastic out of your nozzle," then you might want to follow these steps.
Acetone (get this in the "fingernail polish removal" area of a drug store or grocery store)
7 mm wrench
3/16 inch diameter rod
hex wrench supplied with the Replicator
This is no more hazardous than pulling cookies out of an oven--but you wouldn't grab a cookie tray with your bare hands. Don't touch hot parts until they have cooled to room temperature--be patient.
First, unload the filament from the extruders.
Unscrew the extruder assembly (two screws) and set it on top of the Replicator. Place the 3/16 inch diameter rod between the extruders.
Wait 24 hours.
Take a small drill bit (or a toothpick) and pull out as much "mucky plastic" as possible. Do not drill inside the nozzle, just use hand pressure to gently remove plastic.
Soak the nozzle (submerged) for another 24 hours in acetone.
You should be able to look through the nozzle and see light. If you don't see a clean "hole," then soak another 24 hours and "floss" again. Don't use floss or string--they might get stuck in the nozzle.
Preheat the nozzle to 220 degrees and--using the 7mm wrench--tighten the nozzle in the hot extruder. Don't go macho, this will be 1/16 of a turn or less--you want it secure but not to the point of stripping threads.