I would try two things. The first is thinking back to your childhood and trying to remember the things you really really enjoyed doing, often that's where the spark of your passion lies but often we ignore our passions for so long that we forget what they are. When you were a kid what did you want to be when you grew up??
The second, more drastic measure, would be to take a few months off work and just do nothing for a while. Creativity generally requires rest to function so if your passion - which may be yet to reveal itself to you - is creative, rest will really help. Take it easy and notice what you naturally gravitate towards each day and that should give you a good idea of what you're naturally passionate about.
I hope that helps
