Heatercartridge bolt stuck
Don't Panic!
First of all, a printhead caked in PLA is usually not a death sentence, it is often a temporary setback. Let's start in steps!
Step 1: remove the extruder
We want to work on the hotend, so we remove the extruder feed first. For direct drive, we unload it, then remove it depending on your printer, so the cooling of the hotend remains. For a Bowden, unload and, if possible, remove the Bowden tube from the cooling block. If not disconnect it from the extruder.
Step 2: remove from the carriage
Now that we have the whole hotend assembly bared, we remove it from the carriage. Usually, it is 2 to 3 bolts, more if the cooling solution is mounted separately and has to be removed to access it.
Step 3: remove the cool-end
If you can, remove the cooling fins - we want to have the heatbreak to hold on to.
Step 4: Clamp it up
Take a fire-proof surface (ceramic tile!) and put down the hotend. Rig it up that it can't jump away, best with a small vise. Don't bend the cables!
Step 5: Free the thermosensor
Let's start to clean! If your cartridge still works, get 12V onto the heater and let the stuff melt a little. Use pliers and a scraping tool to clean the thermistor cartridge.
As you have no heat control only heat in short bursts to prevent fire and destroying the cartridge too.
As an alternative and if you can't get the cartridge to work, use a hot air gun or a hot-air soldering station. A soldering iron with a broad tip also works well to scrape off the plastic with controlled heat an as a scraper to remove large chunks.
If you use an external heat source, free the heater cartridge first and remove it, hoping that it is not also dead.
As soon as you can, get the thermosensor out. If you run on the heater cartridge,  install a fresh Thermosensor, even into the goopy heater block, and wire it to the board to regain temperature control and use the board to provide the power.
Step 6: Finish cleaning
I usually clean up the final stretch (and as far as I can: all steps) under board controlled heat:
Make sure that heater cartridge and thermosensor are installed well and working. Set the hotend to 170 to 180°C and do the last cleaning under the use of regulated temperatre.
Step 7: Do steps 1 to 4 in reverse order.
Reassemble, following the steps backwards.
Step 8: Hot Tighten!
Heat the assembled hotend to 240°C, then tighten the nozzle against the heatbreak to ensure tightness. Let cool, done.
Alternative
Replacement/Upgrade!
One could go down to step 3 and replace the hotend assembly (Heaterblock, Thermosensor, heatbreak, nozzle) with fresh parts and reassemble. This is much more expensive than reusing but cleaner and faster - if you need to print now, you might want to keep one hotend assembly on hand as spare and clean the one replaced while the machine runs.
External heat sources
As mentioned, a hot air gun or soldering iron can provide heat to remove the plastic caking.
The soldering iron has the benefit of doubling as a scraping tool and providing localized heat, allowing to possibly free the thermosensor without unsoldering anything in it, and it won't melt the heater block.
A heat gun provides gentle, overall heating of the beater block, but needs extra care where the hot air is going - it can easily char wood and might remove the solder from the heater cartridge.
A gas torch might be used to burn off any residue on a totally stripped heater block, but it also would be able to melt and deform an aluminium heater block! Do not use it on a still assembled heater block, or it will melt any solder in the heater cartridge and destroy it.
In any case, working on a fireproof surface is mandatory!
Chemically cleaning (for non-PLA)
PLA can be removed chemically, but the solvents are rather nasty and some take quite some time to work. Very toxic dichloromethane was used to make a solution of PLA to create thin layers used in this paper, and most other solvents the study mentions are at least equally nasty. The available working options - ethyl acetate and propylene carbonate - have high price tags associated with them. So chemically cleaning PLA from cheap nozzles is not an economically viable option. However, it is a viable option to use acetone if you have your hotend caked in ABS.