Sweetcorn can go pale and sickly when temperatures are too low, including at night, but as you say that's unlikely given the recent weather round these parts.
A liquid feed may well revive them - most if not all of the general purpose liquid feeds are high in nitrogen.
As AnneB says, don't overdo the watering - her suggestion sounds about right to me unless your soil is particularly free-draining and/or the weather especially hot.