Sorry this has happened. Bit strange she went off broody so close to hatching. When Hatching is near, my broodies do not leave the nest, they seem to know. Had something scared her do you think?
One of my broodies was a few days off hatching, she left the nest, and I always check she is back on before going to work etc. well, she was off for quite a while when I remembered she was out. Eggs were stone cold, but put her back on, and 5 hatched out of 7. The other two were duds.