Help! Add to cart only works when logged in

[BlueOvalIndustr]

Hello, so earlier today we all of a sudden had reports of people not being able to add products to their cart.  The last order we received was at around 11:30. 

When 'add to cart' is clicked on either the product page, home page, or category page the cart simply says

"You have no items in your shopping cart.

Please continue shopping."

When logged in I am able to add products to my cart.  

I've read posts about session errors and everything and have messed with all the settings I know.  I upped the cookie lifetime to 86400 & tried turning on Validate HTTP_USER_AGENT. I've re-indexed and cleared caches. I've renamed the var/cache folder so that magento would create a brand new cache folder.

I really have no idea what else to try.

Also in my system.log file I have a whole bunch of errors that say the following:

2015-07-07T19:47:52+00:00 ERR (3): Warning: simplexml_load_string() [<a href='function.simplexml-load-string'>function.simplexml-load-string</a>]: Entity: line 1: parser error : Malformed declaration expecting version  in /home/bluesimp/public_html/lib/Varien/Simplexml/Config.php on line 510
2015-07-07T19:47:52+00:00 ERR (3): Warning: simplexml_load_string() [<a href='function.simplexml-load-string'>function.simplexml-load-string</a>]: &lt;?xml version=”1.0” ?&gt;  in /home/bluesimp/public_html/lib/Varien/Simplexml/Config.php on line 510
2015-07-07T19:47:52+00:00 ERR (3): Warning: simplexml_load_string() [<a href='function.simplexml-load-string'>function.simplexml-load-string</a>]:               ^  in /home/bluesimp/public_html/lib/Varien/Simplexml/Config.php on line 510

[BlueOvalIndustr]

Additionally the "Add to compare" button does the same thing that "Add to cart" does.  The page just appears to reload with no product in the compare box/shopping cart.