It's easier to maintain a fire than it is to reignite a spark....
One of the most important things is both people have to truly want to turn things around.One person cannot be romantic or passionate enough to save a marriage. It's going to require efforts on the parts of both people. Once buy in has been established you open up a world of possibilities.
One way to jump start things is to have a weekend getaway. Sometimes it's just good to be away from your normal suroundings. Whether you find a hotel near the beach or stay downtown in a large metropolitan city it's good not to have to think about anything but each other.
You might start off with a meal in a quaint restaurant followed by either listening to some live music or going to an intimate nightclub where you can dance and hold each other close. (smooth jazz) or something romantic.
When you get back to the room light some scented candles, give each other a sensual massage.... and take things from there. The following morning order room service, afterwards take a bath or shower together.
Once you get home if the two of you are feeling adventurous you can visit an upscale adult store and pick up some things to help spice things up from movies, board games, toys, and various other items.
Generally speaking long before sex "dries up" in a marriage (affection dies first). Couples STOP holding hands, hugging, exchanging (wet kisses), snuggling while watching TV, flirting with one another, using sexual innuendo.... and so forth. The more distant you are out of bed the more difficult it is to connect in bed.
If things have really gotten bad you may need some professional help to get back on track. One very well known expert is Laura Berman who has several books and products which might aid you. Best of luck!