This is possibly a PHP issue either caused by an extension, modification or incompatible PHP version. I would comment but don't have reputation so I am answering instead.
In the browser look at the html source code - this can indicate where the page stops rendering exactly to help pinpoint the issue.
Check the error logs for the webserver you are using (apache / nginx for example) Check the error logs for PHP and PHP-FPM(if using).
Check the version of PHP you are using is compatible with the version of magento you are using - for example PHP 7 will cause this issue.
Try disabling any extensions one by one to test. Do this by changing the xml file for the module in /app/etc/modules/ to be set to false instead of true, clear cache if on afterwards.