This is a very common issue. For security reasons error reporting is disabled from displaying on screen. There will be an error message somewhere and it's only possible to know what the problem is when the actual error message is known.
One of the three steps below should help you find what the error message actually is:
- Login to the admin side of your store (if you can) and have a look at the "System Error Log" which can be found following the "Error Log" link on the navigation menu.
- Edit the classes/debug.class.php file and change the argument from "true" from "false" in the __construct method where an ini_set is used to set the value of "display_errors". Then refresh the page to see an error message on screen.
- Check your web hosting account directory for a php error log file. It can have a number of names but typically it's "error_log". You can find where errors are logged via the "PHP Info" link in the admin control panel of yourt store. Look out for the path next to "error_log". If you can't access the admin side of your store create a file on your server called "info.php" and add the code . If you then view this in a browser you can see the same information the admin side of your store would show.
Some web hosting companies disable PHP error log files to save disk space. If you contact them they may be able to enable them for you or tell you how they can be enabled. There are a number of ways it can be done based on your server configuration.