Sudden 500 Errors after 2.7.1 Upgrade

I upgraded my blog (http://www.aishiteru-tokyo.com) to 2.7.1 and suddenly it seems like nothing works – you can see the front page but all of the links are 500-ing.

When I log onto the dashboard, all of my posts/pages/comments are still there and everything seems to work fine otherwise.

Help?