If you are unable to pull the material out the first thing to check is if the filament is still loaded into the tip.  If so, check the Loss of Extrusion article. 

If no filament appears to be loaded into the tip and you cannot pull the filament out, it could be stuck here: 

In that case, you'll need to push the filament up from the material bay: 

At the head, that should push the filament down so that you can then continue with the Loss of Extrusion procedure to snip the filament off.