Many time when we work on drupal site, when we add modules or do any other work we face lots of problem of memory like we see white blank screen, memory error, page not found. 

I found this kind of errors when my browser cache is full or i worked continuously on project with opening lots of tabs in browsers. many time i face that error and when i closed some tabs, browsers and clear memory cache, the error is gone.

But lots of time we have to increase the memory limit into our php.ini, .htacess and in settings.php to avoid such errors

If you face such kind of problems, errors make some modifications into your following files and check for errors if 

php.ini

max_execution_time = 64M

max_input_time = 64M

memory_limit = 64M

post_max_size = 1M

.htaccess

php_value max_input_time 64M

php_value memory_limit 64M

php_value post_max_size 1M

settings.php

ini_set('post_max_size', '1M');

ini_set('upload_max_filesize','2M');

You can modify the values as per your requirement. If your site contains heavy data, lots of images, lots of modules then you can increase the values as you need.