Sometimes instead of web pages with data, you only see blank web pages.  Some of the reasons for this could be :

1) The PHP code does not execute mid-way through rendering the page server-side so it crashes returning null headers back to the client browser.
2) The server the website is running on has no memory left to process the request , this is an intermittent problem.
3) The  website is running on non-server grade hardware that does not have ECC RAM and/or production grade CPUs  with adequate capacity. So whenever there is memory corruption or bad bits the Apache process fails midway.

In each case, you can try again after some time.