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.