I have central upper tooth with crown. What are risks of my dental crown “sitting” on top of my gum line?

I have central upper tooth with crown. At attempt was made to probe to measure the space under the gum. dentist was unable to insert the instrument because the edge of the crown was sitting on top of the gum. Advised that it will need to be replaced. What are the risks of keeping the crown as it is (on top of gum)? What is this issue technically called?