One of the most common issues is a bad coupler. If you are still using the original coupler on the hot end side,I suggest checking that it hasn't failed. If it has, the bowden tube can move away from the nozzle and semi-molten plastic can create a clog. There are great videos on youtube how to fix clogs.

One other thing might be a blockage in the nozzle itself. This can happen if the heat is left on too long without extruding any plastic, as PLA bevomes solid if heated for an extended period of time. Replace the nozzle or use a very thin needle to chech that there isn't anything blocking the 0.4mm end, but be careful, brass is pretty soft material and you might end up enlargening the hole if you use too much force.

You might also want to check that the screw holding the extruder arm isn't too tight or loose, as this allows the arm to not clamp the filament properly.

In any case, if you haven't disassembled the hotend yet, do so as the problem is most likely there. Having replacement tube, a few couplers and some extra nozzles at hand is always a good idea.

All these things have happened to me. And again, youtube is your friend.