Septic tank was not serviced. Please remember, septic tanks are nothing more than solid-separting devices. If they are not serviced or cleaned when needed, they will lose the ability to separate the two and can cause for a system failure and could lead to needing a new system.
Septic system replacements in our area can cost anywhere between $8,000 and $25,000 depending on the system requirements.
Here you can see that the solids have completely packed the pipe here. This is after the septic tank and in the drainfield.
Servicing the septic tank can avoid this costly repair.