Here's some good exercises to help get out of any kind of writing rut:
1. Put on a movie that you're familiar with and watch it with no sound. Then try writing a song to a certain scene.
2. Try to start your first line with a fact. (The sky looks different today.... My car just died --- wait! not for you ... how about ... I bought a new car! .... I used to have a dog .... etc.) After that, you can wander to wherever. When you do this, it will usually get you thinking in different parameters.
3. Do lots of freewriting. Just fill up pages and pages with the first crap that pops into your mind. If you can't think of anything, then write "I can't think of anything" until you think of something else. Don't worry about anyone else ever reading this stuff; it's just an exercise. Many times when I do this, I'll scan through it and find a line that I really like which will spark a whole new song.
Hope this helps.