In the proposal editor, flip through the pages using the 'Current Page' drop-down menu and make sure the 'Page Visibility' option on the left-hand side is set to 'Visible.' This will restore your page for you to edit and include in your Proposal!
Once you have done so, you can click the 'Save & Publish' button at the top of the Proposal page to update the Proposal PDF for your client to see.