You may have to cut the floor all the way through, Drill #4 re-bar 9" into the existing floor every foot around the hole, insert with epoxy. Make sure the bottom is dug down to undisturbed earth, no mud. Add 4" of limestone to keep the water down away from the concrete, cover with 6mil plastic. Now lay a mat of #4 bars in the hole with a bar eack foot in each direction (1' squares) and use 4,000# rated concrete to fill the hole. Use a 16d nail to rake around the hole between the new and old about 1" deep while it is still wet, cure the concrete and fill back with silicone caulk.