In older versions of chrome, there was a button to "restore removed thumbnails", but I can't find such an option in Chrome 15. When I click the "x" at the top-right of a most-visited site thumbnail, I want the ability to bring it back in the future.
When you click the "x", on the top of the page (screen), you should see a message popup that reads "thumbnail removed" then gives you the option to undo or restore all.
If you've waited too long, and that bar has disappeared, you'll need to click "restore all" to get your thumbnail back.
EDIT: Because of a recent update to Google Chrome, this solution, as mentioned by Muxicoid in the comment below, does not work anymore.
The only way now to restore a site is to remove another thumbnail and then click Restore all thumbnails when it pops up along the top of the page.
For me, I had the site saved as a bookmark on the favorites bar - then I took mouse and dragged this over an existing frequent icon that I did not care about. Seems to work.
I encountered the same problem before where some of the most visited page thumbnails won't display..they're just blank white! The easiest way to solve this problem is by sign out from chrome and sign in again. Believe it or not..everything just went back to normal