This happened in our kitchen sink, dual sink that came down to one line, was grease in the trap from drained meat (a no-no on a septic anyway), wouldn't plunge, trap was solid grease. Good luck!
Probably what it is. She said it was running slow yesterday and she plunged it and it seemed OK. It has happened before and their hired man would take the trap apart and snake the line out. Yep, septic system out on the farm.