This is usually caused by a PHP fatal error, check the page source and PHP logs.
The most common fatal error is an out of memory error. Check the Statistics->Optimization page and see how much memory is currently assigned to PHP and how much the overview is using.
If it is a memory issue you have two choices:
- Increase PHP’s memory allocation
- Delete some of your historical data.
See http://php.net/manual/en/ini.core.php#ini.memory-limitp for information about PHP’s memory limit.
To remove historical data you can use the Statistics -> Optimization -> Purging -> Purge records older than.