Suggestions:
-Build a song on someone else's text. Could be a novel, poem, love letter whatever. I did a little project with short horror stories. 1 page stories do well for usual 3-7 minutes songs. Or longer stories you can divide in chapters (2-3 songs for the whole story) or 40 minutes song a la Edge of Sanity Crimson 1-2.
-Porcupine Tree's Blackest eyes, song is on a serial killer burying people in his garden. It could be from a newspaper article as well a lot of people are inspired by the news. (sunday bloody sunday comes to mind... )
-Start a song from a different sound on your favorite instrument.
-Start from a melody.
-Start from chords.
-Start from an acapella blabla from a singer.
-Start from the riff you found just fooling around on your favorite instrument.
Keep it simple and be sure to have fun !!!!!