PLA Filament stuck after printing with Prusa ABS-T - Nothing extrudes, unloading not possible.  

PLA Filament stuck after printing with Prusa ABS-T - Nothing extrudes, unloading not possible.

I have been enjoying my Prusa MK3S and all my prints went well so far.

Today I wanted to try out ABS filament for the first time. I have Prusa ABS-T in white and the item printed ok.

After the print I unloaded the filament, it unloaded normally.
Then I loaded some colored PLA right away, and the white ABS-T was extruded, I could not yet see the PLA color so I answered "no" to make it extrude a bit more of the white ABS, then I answered 'yes' and went to load a new file onto the SD card. Came back after 2 minutes and wanted to start a PLA print. Nothing came out of the nozzle at all, not even a tiny bit of filament.I went into the menu to stop the print.

Then I ran a self test, everything was ok.

I thought to myself, maybe there is still ABS in the nozzle and the temperature of the PLA print is not high enough to get it out, so I started the previous ABS gcode again, hoping it would extrude, but again nothing came out of the nozzle whatsoever. I stopped the print.

I tried to unload the PLA filament, the menu told me to unload immediately, but I could not pull the PLA out, I pulled a bit harder, no chance to get it out.

Now I would like to ask here what to do. I never changed or cleaned a nozzle and I might not have the right tools for it. This is the very first issue I encountered with my printer so far.
I hope somebody can explain what might have caused this and tell me how to fix it, maybe some links to instructions.