Because you don't specify what "The site" is and we don't know anything about it, you just have one possibillity:
- Open your Browser
- Type in your address bar http://google.com
- In it's search field type in "Warning: Creating default object from empty value" + "your Application name xy"
- take a look at the first 20 entries and try some solutions you can find
In most cases this will be an incompatibility of the application with PHP 5.4.
Next time, please give us much more detailed in your posting so that we can give you much more specific support.