Here is a quick list of romantic restaurants, but you may already have trouble getting a reservation for Valentine's.
Tru - Very expensive, but one of the best restaurants in the world.
Ambria - classic French in a great location in Lincoln Park - expensive
Everest - Very expensive French, on the top floor of Loop building (Chicago Stock Exchange)
NoMi - in the Park Hyatt on Michigan Ave - great view of the Watertower.
Coco Pazzo - maybe the most upscale Italian around.
Geja's Cafe - fondue; very romantic, sometimes an older crowd.
Tizi Melloul - not as much of a dress-up place, but exotic and romantic Middle Eastern
Signature Room - on the 95th floor of the Hancock building, but the food is not as good and this is kind of a tourist cliche place.
Cite - best view. In Lake Point Tower (Navy Pier). I've heard the food is much improved which is good because it used to suck.
Il Mulino New York - another formal Italian place in an old Gold Coast Mansion - very romantic, fairly expensive.
There are plenty more, but look into a few of these and see if there is something you like.
If you want romantic and trendy, club kind of place look into Carnivale or Sushi SambaRio - these are not quiet romantic they are night on the town kind of places.