Assign yourself a goal, like a writing exercise. Decide to write one in a certain mode or scale, or maybe one that modulates to another key halfway through or on the chorus or bridge. Or pick a topic, or word, or phrase at random by whatever method you can come up with (close your eyes and stab your finger at the newspaper or throw a dart or turn on the TV and use the first phrase you hear, or the first billboard you see wnext time you're driving, etc.).

Things like that can kickstart an effort. Creativity is a psychological and emotional muscle. You have to exercise it.