If the system has worked in the past, it sounds like a clogged line. If under garbage disposal a clean out plug was installed, I recommend running a snake to unplug the clog. If you don’t have a clean out, you might have to remove the garbage disposal to get through the line. If your having any other problems in the home, it could be your septic tank is full (if you have one) or a venting problem. Being its a sink with a disposal, its more then likely a clog or abstructed line.