LianeD, please provide more detailed information regarding exactly what structure on the septic tank has been damaged. Is the tank made of concrete, or is it a plastic or fiberglass tank?
Typically, the top of a septic tank is buried. In some cases, concrete septic tank access hole risers are installed to the ground surface. If the concrete riser lid is “chipped”, the damage may be repaired with epoxy cement. If there is a hole in the lid, and sewer gasses are emanating into the yard, then perhaps it is best to replace the riser lid. It is potentially a very dangerous situation if the damage to the tank is severe enough to compromise the structural integrity of the lid. If someone should stand on the lid, and the lid collapses, death could result for that unfortunate individual. What a terrible fate.
Until you provide more details about the situation, I can not provide more advice.