If this happens a few days after making your first starter then just keep going with your feeding schedule. It's normal to get a burst of sudden activity and for it to calm down for a while before things really take off. If this happens to an established starter though, dispose of all but a really small amount ( just keep a teaspoon or two of it) and start daily feeding with rye flour as per my instructions for making a new starter. It should soon perk up!