1. Always check you debug.log inside wp-content. It will tell you the exact 
issue that is causing this issue.

2. WordPress Memory Exhausted Error – Increase PHP Memory. You need to paste 
this code in wp-config.php file

define( 'WP_MEMORY_LIMIT', '256M' );

reference solutions:
https://www.wpbeginner.com/wp-tutorials/fix-wordpress-memory-exhausted-error-increase-php-memory/