Yeah, snails will sometimes sit for days inside their shells. When they do that it usually means they're stressed about something, in your case its probab;y water quality. You said you have a 10 gallon with two goldies and two snails? The goldies need a minimum of 10 gallons PER fish, snails need 2-5 gallons PER snail. For your load you really should have a 25-30 gallon tank. Being that overstocked causes water problems, I'd suggest doing a large water change and see if that helps your snails out any.
They should be fine until tomorrow without food, snails can be pretty tough critters

As for feeding, just find a way to safetly clip some lettuce to the side of you tank, or find a way to weight it down, maybe put a corner of the lettuce underneath the corner of your tank ornament. www.applesnail.net is a great place to learn about your snails, pretty much the whole forum is dedicated to the type of snails you've got