Barbara,
To reset your preview so the cover shows on the product page, do the following:
1) Click on the title on your MY LULU Page
2) Scroll down to the Preview section of the revise project page and click on the Edit button (you can also get to this by clicking on the Customize Preview link when viewing your preview page)
3) For a simple preview that will show the book cover and the first 10 pages of text, leave the simple option button selected and click the Save & Continue option. You can also choose one of the other options if you desire
4) Once the preview generation completes, click the Save & Finish button
5) Preview should now include the cover.