It may be that the main site front end works fine, but the admin area is error 500 or just blank depending on browser.
Turn on debug mode to try and find cause of problem. In “/wp-config.php” find (or add) “define(‘WP_DEBUG’, false);” and set it to true.
This may reveale a memory size limit issue.
If so, to fix paste (or edit if present) this line into wp-config.php file
define( 'WP_MEMORY_LIMIT', '256M' );
Other things to try
General recommendation of temporarily renaming /wp-content/pluggin directory and theme directory (after ensuring a default theme was also present).
Download fresh wordpress and copy files over top of existing site outside of wp-content.