A cover says everything about your book, the more simple, the better. It is what draws a person to pick up the book and read the back cover to find out more. The same goes for a title, simplicity the key.
If what you want to write doesn't come to you, walk away. Even for fifteen minutes or so. Do something that isn't stressful. You'll find when you return, the sentence you'd struggled with or the words you couldn't find, are now right at your fingertips.