Maybe add a little water to it now, and let it set. (a cup or two, depending on how much pasta). Pasta can still absorb water when it's cold--think of how the noodles expand in leftover chicken noodle soup. Give it a stir now and again.
Sometimes, cooks deliberately leave the pasta a little under-done, so it'll absorb the liquid/flavors of whatever they sauce it with. It's a technique! Probably has its origins with someone like you (who accidently undercooked the pasta), but hey--go with it! You PLANNED this, right?
If all else fails, blame the recipe.