The printer starts fine. Then it stops extruding while the head keeps moving, and often the extruder clicks as it tries to push filament into a hotend that will not take it.
What is happening
Filament should melt just above the nozzle. When the heatbreak above the heater gets too hot, the filament softens higher up, swells and jams. The extruder then grinds or clicks against it.
Heat creep is one way a hotend clogs, and Prusa notes it is often not the most likely one. Rule out an ordinary clog and under-extrusion as well.
Normally filament melts just above the nozzle. With heat creep it softens in the heatbreak and jams.
Common causes
A warm room or a closed enclosure. Prusa gives above 35 °C, or 30 °C for some filaments, as a risk, and PLA is the most prone.
A heatsink fan that is dirty, blocked, slow or mounted backward.
A hotend temperature set higher than the filament needs.
Poor heat transfer between the heatbreak and the heatsink, such as missing thermal paste after a rebuild.
Too little filament moving, from very thin layers or slow printing, so the filament sits in the hot zone too long.
Filament with metal particles, which carries heat up the hotend.
Fix it
Clear the jam
Start with the jam. Follow your printer maker's clog guide or do a cold pull, because heat creep leaves a clogged hotend and nothing else helps until it is clear.
Let heat out
Open the enclosure door or top cover when printing PLA or PETG, as Bambu Lab advises. Lowering the bed temperature for PLA by 5 to 10 °C also helps, according to Prusa.
Clean the heatsink fan
Blow dust out of the heatsink fins and the fan. Check that it spins. It must blow toward the heatsink, not away from it.
Keep filament moving
Use thicker layers or print a little faster where detail allows. After a print, unload the filament while the hotend is still hot.
PLA softens at a lower temperature than the other common filaments, so it jams first when heat climbs up the hotend.
Heat creep lets the print start normally, then stops extrusion partway with no error while the printer keeps moving. A nozzle set too close to the bed shows from the first layer.
It can contribute. Prusa suggests lowering the bed temperature for PLA by 5 to 10 °C, and adding a brim if the print then struggles to stick.
Specifications come from manufacturers' published pages and can change. Check your printer's current documentation before you change a setting or fit a part.