Broken/busted main cast iron drain line
As others have suggested, it might be a good time to call a pro, but if you are going to handle it yourself, here is what you should do:

see if you have enough space above the slab to fit a rubber collar of the cast iron/pvc adapter. If this is the case, you should secure the cast iron pipe and cut it. Replace the section with PVC and put rubber adapters on both sides.

If you are not lucky and you don't have enough room, you have to break the slab (usually 4" max) and get to the sand/gravel layer. The main stack should make a 90 degree turn. You should place your cast iron to PVC adapter there. It is just as much work as it sounds like, but it can be done.