I found a hole in the floor of the fireplace. It extends the width of the fireplace, just between the fireplace floor bricks and the bricks of the hearth in front. Can't tell how deep it is because I don't have anything long enough to reach the bottom. I am concerned. Please advise.

Is this a prefab fireplace? If so, the fireplace floor is probably an insert that is removable. You probably have a fan, the insert would be so that you can service the fan if necessary.

Harold asks a good question, is it prefab? However, even prefabs do not have this hole so a picture would be good. Right where you describe a prefab has a fire stop plate installed to prevent ember from getting done to the framing.