It sounds as though your dahlia tubers got too dry. Always make sure that the roots are slightly moist before putting them in storage. The wood shavings you used are a good choice, as are vermiculite, perlite, dry sand, or peat moss. Keep the tubers cool in storage, but above freezing (40-45F) is ideal. Check them regularly during the winter, and sprinkle a little water on them if they begin to dry out. A layer of newspaper over the top of the box will help hold in moisture. If your dahlia tubers are completely shriveled, they may be beyond salvaging. If there’s any fleshiness to them, you might plant them and see what happens.