I had set a print overnight, and this morning Repetier said it was finished but only about 90% of the layers had been printed.  After some consternation, tried to print something quicker and easier first, and noticed no extrusion at temperature.

According to http://www.solidoodle.com/how-to-2/troubleshooting/
they advise trying to extrude to push through,
but unfortunately all that did for me was crack some acrylic parts in the stock print head
:-(

Luckily, so far the guitar-string cleaning method has allowed plastic to extrude once more, but without knowing what caused it, I'm a bit worried that subsequent prints are subject to the same issue (and this suspicion was abetted by the next print running into the same issue printing its skirt)

any advice?  I have already printed a replacement extruder set for when the acrylic actually breaks ;-)