What'd I'd suggest is thinking of it as a form of poetry and not forcing the hand of the music to fit the lyrics, or forcing the lyrics to fit the music.
It seems that the best lyrics are written on a topic that you feel interested in.
So, once you have the music, I'd say this is a good starting point to find the topic you want to talk about. Take the music you wrote and ask yourself "What kind of mood is this in? Is it happy/upbeat, sad, energetic, slow, intense, laid-back?" Once you have the right idea about that, you can think of a topic to write about.
As others have said, it is best to just let it come to you, but I remember being in the very same position as you and saying "nothing's coming." I would say that isn't a good reason to give up on it though. You should be careful and give it a lot of thought, but don't be too careful or give it too much thought. Just try to fit exactly what you think the song would be about.
For instance, one of my songs is about being a teenager (as if that hasn't been over played already...haha) Anyway, I wrote the melody and chords before I wrote the lyrics. And when I looked at what I had made, I said, "This is simple, but sort of skewed. Sounds like some one who is making a faux-folk song. It sounds like it would be about conflict although the music implies unity. What things do I know that are about conflict?" I thought of war, relationships, Man Vs. Nature, Man vs. Self, and then I came along to being a teenager. Although I was a teenager at the time, it wasn't so obvious that's what it should be about. So I used the mood and rhythm of the music I wrote to come to a good topic, and then used the topic to find specific line by line lyrics.
It's easier than it looks at first. Don't worry about the lyrics being dull or bad, because it is a learning process. Unless you write poetry or creative works, it doesn't seem like the first song you ever write will be the best of your songwriting endeavors. But that is how you learn. And you'd be surprised, it's not as hard as we make it out to be. If you have something worth saying, and something worth playing, the words are sort of easy.
If you'd like, I'd be more than willing to help you out with some specifics, just message me or something. Good luck!