Put the site into dev mode - by adding ?isDev=1 after the URL or by setting Director::set_environment_type("dev"); in the mysite/_config so you can see the proper error not just the generic error screen :D
FATAL ERROR: Object::__call() Method 'SideBar' not found in class 'BlogHolder'
At line 190 in /usr/local/www/apache22/data/sl/sapphire/core/Object.php
user_error(Object::__call() Method 'SideBar' not found in class 'BlogHolder',256)
line 190 of Object.php
line 82 of BlogEntry.php
The 1st line I am sure says it all, but I have done nothing to my site except to make the small edits so that I can place Widgets on all of my pages as needed.
Let me know if you would like me to paste the entire error page.
I sure hope this is an easy fix ... I have put a lot of work into this over the weekend.