As wordpress use php and the server needs sufficient memory to run the website / application.
By default wordpress tries to increase the size of the memory by 64MB if the allocated memory is less that 64MB but this memory sometimes is not sufficient to run the application.
Sometime due to lack of memory it show the following error.
Fatal error: Allowed memory size of 41943040 bytes exhausted (tried to allocate 20480 bytes) in /home/sitename/public_html/blog/wp-content/plugins/includes/filename.php on line4299
How To Increase PHP Memory Limit In WordPress
First of all you need to open the file wp-config.php located at the root location where your wordpress is installed.You can use any FTP client software or C-Panel to open this file. Now its time to have fun after updating this file by given single line code.
define( 'WP_MEMORY_LIMIT', '256M' );
Now wordpress conifugration file read this code and increase the size of PHP Memory.
If this single line code doesn’t work for you you can contact your server administrator or you can also edit the php.ini file on the server.