<p>I didn't have any printer-related problem for the past 6 months, but now all of a sudden my Prusa MK3S stopped extruding during printing.</p>
<p>This is very strange as I can easily load\unload filament and control the step motor via Settings\Move axis\Extruder. When I did so, the filament got extruded normally.</p>
<p>However, when I try to print something or do First Layer Calibration, nothing comes out of the nozzle. I tried changing the Live Z (maybe nozzle too low) and it didn't help. I once managed to extrude <em>something</em> by increasing the temperature and the flow (in the printer's menu) to a ridiculous value of 999. Obviously, this isn't the best way to solve the problem.</p>
<p>Is there an easy way to fix this? I only had this problem yesterday and with some midrange-priced PETG</p>
<p>Edit: I tried different filaments, default slicer profiles, reinstalling slicer (prusa slicer) and drivers. None of these methods really helped.</p>

It doesn't seem to be heat creep. See
What are ways to avoid heat creep?
Have you measured the actually temperature of the heater block?  You may have a failing sensor (thermistor) or sensor circuitry.  Optically is the best way to measure.  The least expensive way is with multimeters that come with a temperature sensor, such as a thermocouple (lowest cost about $20 U.S.).