Wifi shared from Macbook comes and goes
I suggest you restart your box to make sure that it’s just not a problem there. You could technically turn off the AirPort and turn it back on (that’d restart most of the stuff), however the “problem” could be on the Kernel level, which would only be fixed with a proper restart or even a hardware buffer, which a cold shutdown would most likely fix. 

In any case, if you still want to experiment, try changing the Channel from Automatic to something you can use in your country and move it, maybe somebody (neighborhood?) brought new devices or networks next to you and that causes new interference that wasn’t there two days ago.