Tracey, I've never had that exact thing happen, but I have had pages get messed up. I can only guess that the underlying HTML code of your page got corrupted - probably there is something in there that didn't get deleted properly. I'd advise against using the "undo" menu item as I find it to be inconsistant (sometimes it will work as expected, and then other times it will rewind the page many many steps - and unfortunately, redo doesn't work at all).
The only sure way to fix a page is to start a new page and rebuild it from scratch using the corrupted one as a visual guide - this is why I advise against using "undo", because if "undo" takes you back too far, you may lose the visual guide that I was refering to. This may seem to be too much trouble to rebuild an entire page, but if you are new to the application, it will get you familiar with building pages. I'd also strongly suggest that you duplicate (back up) your project folder in finder before starting an edit session, because it can save you if something gets corrupted. Once you know that your new edits are good, you can delete the backup.