This is known as the "White Screen of Death" and is most likely caused by a lack of memory on your host.
Increasing the Memory Limit
First open your wp-config.php which is located in the root WordPress directory. Then add the following line inside the main php tag:
define('WP_MEMORY_LIMIT', '96M');
We just increased the memory limit to 96M. If you still get the error after this fix, then please contact your host. Most likely, they would have to go in their php.ini file to increase the memory limit for you.