I'm guessing you are using Internet Explorer as your browser. If so, here is a solution I've posted before (there have been several threads about this issue)
LJ's had the back arrow problem before, it has to do with how doubleclick.net puts ads on the page-the site loads the ads as separate pages, instead of embedded in the LJ page. Easiest way to fix it if you are using IE is to go to Settings, then Internet options and the Security tab. Click on Restricted Sites and then the Sites button. In the "Add this website …" line put in:
*.doubleclick.net
Just like that, including the asterisk, and click the Add button. Reload the LJ homepage and the back button should function correctly after that.
If you are using Chrome or Firefox, then the easiest solution to add the Adblock Plus plugin.