Site throws 500 error after upgrading from PHP 5.6 to 7.X

try doing wp_debug true and
if the issue is of memory allocation try allocating memory in the wp-config

define('WP_MEMORY_LIMIT', '64M');

If it is not fixed try increasing the memory limit.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)