Q. Is it possible to revert changes I have done to a page layout or master page in SharePoint Designer 2010?
A. Yes. Right-click the page layout or master page and select the Reset to Site Definition option from the context menu to revert the changes done to a page layout or master page.
Q. The site is giving access denied errors for most users after changing the default master page to a newly created master page in SharePoint Designer 2010.
A. Make sure that you have published the master page or the master page will not be available to other users and the site will give an access denied error for all users except the one who created the master page.