The problem could be caused if you make some upgrades between Sitefinity 4 version 910
We have changed the location of the CSS file for the backend menu. It seems that with your upgrade this was not properly upgraded in the database. For example if you want to upgrade from Beta 2 to RC 2 you should upgrade to each version in between those two. I have attached an aspx page which you can add to your project root. Then authenticate with an admin account and load the page. This should fix the menu.
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