"Page Cannot be Displayed" Error After Editing for More than 10 Minutes

After editing a book module page (the HTML for in the body window) for more than 10 minutes and clicking the submit button, I get a "This page cannot be displayed error." If I click the refresh button on the browser, I get the page in editing mode back but all my edits are lost. Does anyone have an idea of what is behind this and how to fix it?