usually the on the feedhorn only thing that will go bad is the servo it changes the polarity and also the skew adjustment.
now if the lnb's go out it will react different ways from no signal to off & on signals to fading signals.
a lot of times wasps build their nest in the feedhorn blocking and reducing the signal. and also over time a small drift in the dish setup could degrade the signal. poor connections or bad coax will also degrade the signal quality.
first check the feedhorn and verify nothing is in it blocking the signals
next check all connections make sure they are still good.