I have a sink in the main bathroom upstairs that will not drain. I have replaced the s trap under the sink, poured down two bottles of lye and one of the works and had it snaked. It now leaks from the main pipe in the basement any time someone forgets and uses the sink. I am on disability and do not have much money for repairs. I don't know where to turn to or who to turn to. I'm terrified that I will have to replace the main stack. All pipes in this house are galvanized and I have all ready replaced a 3 foot section over by the water heater, replaced the hot water heater and had new insides installed in both toilets. When the toilet upstairs is flushed it bubbles up and you can hear it in the lower level toilet.

It now leaks from the main pipe in the basement tell me more about this pipe. Is it a horizontal drain lines or is it a vertical stack? Is it leaking from the joint? If so, which one?

When the toilet upstairs is flushed it bubbles up and you can hear it in the lower level toilet.
You have more than one issue. But both problems can be repaired by snaking the d I lines. Make sure that you have a server snake long enough to reach the sink to the basement and 15 feet beyond .
You also have a blockage downstream from the Lord toilet. The solution would be to pull the toilet and snake the drain line from there. Let me know how you make out, good luck, Tom