1) Check permission of files/folders.
2) Increase memory_limit to something higher value (128MB should be enough).
3) Try to repair database through cPanel > MySQL Databases page.
4) Comment a php declaration inside of the .htaccess file.
5) Check error_log file for errors which is present in document root (e.g. /home/$username/public_html/).
6) Try to disable every single plugin/module for your WordPress website.
7) Check for any additional .htaccess file at the /home directory for PHP declaration something below then please remove or comment it. :-
# Use PHP55 as default
AddHandler application/x-httpd-php55 .php