One of my Silkie hens would not get off the nest either... I don't think I ever saw her off the nest. She did make a mess in the nest and we cleaned it out as we found it. It was colder and I wondered if that was why. She hatched every egg! My other Silkie that got on and off hatched 3 out of 7 eggs, so when it's cold, I'm thinking the fact that she is staying on is a good thing! We offered food and water for her each day. We just held it in front of her. Corn will help her produce body heat, so if she is only eating a little, try feeding her the cracked corn.
Try not to worry... I was concerned too... but she did great and we got a wonderful hatch. I hope you will too!