Some of our clients experienced an issue with an update in Google Chrome 45 introducing a bug that breaks the WordPress admin menu.
1. Visit chrome://flags/#disable-slimming-paint in Google Chrome
2. Click the enable link under “Disable slimming paint”
3. Click the Relaunch Now button at the bottom of the screen.
Details and how to on fixing the issue temporarily found on the awesome http://wptavern.com/a-bug-in-chrome-45-causes-wordpress-admin-menu-to-break