If you have the xCDN module and it is active it is possible the "address book" popup not to load properly causing the site to either crash or behave strange. To fix this issue follow these simple steps:

1. FTP to the site.
2. Open /skin/YOUR_SKIN/customer/main/address.tpl file.
3. Near the top of the file find and copy these 2 lines of code:

   {include file="check_zipcode_js.tpl"}
   {include file="change_states_js.tpl"}

4. Comment them out so they look like:

   {*include file="check_zipcode_js.tpl"*}
   {*include file="change_states_js.tpl"*}

5. Open /skin/YOUR_SKIN/customer/main/address_book.tpl and paste the above copied code at the beginning of the file.

NOTE: Make sure to clear the cart cache for the changes to take effect. You can do this from admin - Tools->Maintenance menu link.