PLA broke off at the top of nozzle shaft
I recently installed a .4 hardened nozzle and everything seemed to go well, however my first print got weird with it towards the end. In my troubleshooting, I unloaded the filament - it's certainly possible that I botched the process and caused the filament to break off right at the top of the nozzle shaft. It absolutely will not budge. I tried the process of manually shoving a section of filament with the nozzle heat cranked up to no avail. I finally just pulled the nozzle and put the stock brass one back in for now. Any advice on how to extract the PLA?
Also, based on the print that got weird (missing the walls in a section then eating up the top at the very end of the print), is there a chance my nozzle is messed up? 
RE: PLA broke off at the top of nozzle shaft
This is a multi step process but I have used this on the core1 + stock nozzle and also on my D-Bot with a micon eme extruder
With the nozzle out and held on the shaft with a vise grip (not too tight or you can collapse the tube). Carefully drill out the plastic in the upper part of the nozzle. A 1/16" drill is a bit undersized at 1.55 diam. An alternate is #50 drill (1.78mm) which may be too tight. Take you drill bit and insert it into a known good nozzle and mark off the depth. Now drill out the bulk of the plastic.
Heat the nozzle with a heat gun (I used Milwaukee on the highest temp setting). This will only take about 10-20 seconds. If it is smoking it is too hot. Now push needle in from the bottom several times. After this while still hot, manually ram in some new filament (I keep end roll scraps for this purpose. It is good to use a contrasting color. I have done this now on the 0.4mm and 0.6mm high flow nozzles .
Let it cool down a bit and do a cold pull. You may need to do this a few times. The cold pull will need to have 3 strands at the end for the high flow nozzles. If not, then repeat the process of heating and cold pulling a few times.
If you look at the photo the top piece in the photo it clearly only has 2 strands. Same for the middle 2. I changed filament for the 4th pull and this was successful. Note that a standard non-high flow nozzle will only 1 strand will be at the end.
This can also be caused by heat creep in the nozzle on a long print. Make sure the thumb screws for the nozzle are tight (esp the lower one) .
Now take some cleaning filament an ram it through for 10-02" or so using the load setting on the printer but on the PETG temp setting (or whatever was the hottest temp filament you used prior to this . Look for contrasting color bits and keep feeding until the result is clean. Then unload. Now reload at PLA settings.
I have had this happen when using PLA after petg, TPU or ASA. PLA following TPU is a common cause of clogging. It is a good idea to run the cleaning filament thru at the high filament temp prior to switching to a lower temp filament. I do keep a separate non high flow 0.6 mm nozzle for ASA and PC-CF