No, a jet airliner would not stop in mid-air if it encounters a headwind with the same speed as its own airspeed. The situation you describe is an extreme hypothetical scenario where the headwind speed matches the aircraft's airspeed.
In reality, the aircraft's speed is measured relative to the air mass surrounding it. So, if the aircraft is flying at 600 mph, it means that its airspeed (or groundspeed) is 600 mph relative to the air mass. If it encounters a headwind with the same speed, the airspeed of the aircraft relative to the ground would be zero. However, the aircraft would still be moving within the air mass, maintaining its altitude and relative position.
It's important to note that the aircraft's airspeed is measured in relation to the surrounding air, while its groundspeed is the actual speed at which it's traveling over the ground. Headwinds or tailwinds affect the groundspeed but do not bring the aircraft to a complete stop or make it hover in mid-air.