If your Photo Cart is suddenly getting a 500 error or only displaying a blank page then you or your hosting company has updated the PHP (programming language) to PHP 7 which Photo Cart is not compatible with (and will not be compatible with) mainly because of the removal of the my_sql functions which are replaced with my_sqli functions.
You would need to have the PHP version downgraded to 5.6 in order for Photo Cart to work. If downgrading the PHP doesn't work then you may have to update Photo Cart to the last version of 7. Download Photo Cart 7 upgrade files. Upgrade instructions.