If I don't have anything specific to work on, I will usually work on a freewrite for awhile. This means I take pen and paper and write down everything that comes to mind, no matter how random or inane. If I have music running in my head, I write down the lyrics. If I don't know what to write, I say "I don't know what to write." Eventually some idea always develops on the page, and I take it from there.