You would need to replant this year anyway. Parsley is biennial - the first year it makes a lot of foliage but the second year, new foliage growth is sparse and the plant focuses on flowering and seed production. And the leaves tend to have a much stronger and more bitter taste as well.

Although parsley enjoys moist soils, it does require good drainage. It may be that excessive rainfall made the growing conditions too wet and the plants just rotted.

OK Thanks gardengal I did notice it seemed to be bolting already.