I have a toilet in my downstairs basement that won't flush. I had septic tank pumped and even replaced toilet. Still won't flush. Why?

Septic doesn't usually enter into flushing unless both tank and field are clogged. Only the downstairs toilet? You probably have a blockage between that toilet and the septic tank.

I have a toilet in my downstairs basement that won't flush.
"Won't flush" covers a lot of ground. Does it back up and overflow? Does if fill and simply swirl around? N What? Please explain. Back to you, Tom