It can be a tough question. And it requires some digging to get to the bottom. An important one though without a doubt.
What truly fulfills you?
I've been reading Csikszentmihalyi's oevre on "Flow" (What a name, right? I had to copy and paste his name :D The book's great though). He gives important clues as to what truly fulfills you. He does so with scientific backing and concrete applicability in your day-to-day life. It involves the crucial distinction between enjoyment and pleasure.
You can engage in a known activity that you know you enjoy. Like eating, unless you happen to be breatharian. Or sex, unless you... Well really, who doesn't like sex? It gives you pleasure. But the degree of fulfillment you get through eating or sex can decrease the more you engage in it. We've all been there. The same food that delighted with its delicious taste when you ate it first, get's pretty boring when you eat for the fifth time in a row. Or many couples who used to be so madly in love with each other and who used get such a thrill out of the intimate act of mating, loose interest on a sexual level at some point. Been there, done that. Or her. Or him.
By now you could've guessed it. The secret ingredient is novelty. This is why we feel like having pizza and, for once, throw a slice of pineapple and some tuna on it. Or maybe that's just me...
This is why a man can be in love with his girl friend, and still starting an affair. This is why a woman although generally happy with her lovely, generous and attractive husband, can feel the desire to have an exciting, sexual adventure with an other man.
To add something new to our lives can turn simple pleasure into enjoyment. The fact that we stepped out of the comfort zone into the unknown eventually leads to more, what I would refer to as, "Aliveness". We're getting a lot closer to what others refer to as "Flow". This mix between the leap into the unknown and enjoying the experience is unbeatable!
If you feel like you lost that spark, ask yourself:
How can I add something new to my life?