The German offensive pretty much cut the Allied forces in half. Trapping the northern half in Northern France/Belgium and pressing them against the coast of the British Channel. For reasons still not totally clear, Hitler stopped the advance for a few days. This allowed a huge number of British troops (more than 300.000) to be evacuated to England across the Channel. While this was a huge victory for the Germans, it was also a huge deal for the Allies, as it allowed Britain to keep its manpower out of imprisonment.

There's two reasons why the troops could not simply turn around. One was that the German push through the Ardennes came as a surprise. When it became clear what was going on, it was too late. Nobody expected this. While this was going on, the Germans were also attacking through Belgium and the Netherlands as well. The Allied forces in Northern France and Belgium were engaged in this frontal assault as well. Had they turned to face the southern German push, they would have been steamrolled by the Germans as well.