If you try to open modules page: Administer > Site building > Modules and get the blank page instead of it, then you probably have a problem of memory lack. When this page opens, all system modules are loaded regardless of if they are enabled or no.

There are two ways to solve this problem:

1. Remove (unused) modules — fast and simple solution.
2. Increase PHP memory limit changing/adding one of the following lines.

In php.ini file:

memory_limit = 32M

In settings.php file:

ini_set('memory_limit', '32M');

In .htaccess file:

php_value memory_limit 32M

If you have the appropriate access rights, we recommend you to use php.ini file. Depending on how much what modules do you use, you may need to increase memory even further. Determine the required size experimentally.

All fatal errors can cause appearing of a blank page. Therefore you should view server logs and search the similar line there to make sure that your problem is in memory lack:

Fatal error: Allowed memory size of 8388608 bytes exhausted (tried to allocate 418591 bytes) in /path/to/drupal/includes/database.mysql.inc on line 29

This line says that Drupal requires more memory. Do not forget to restart Apache to apply your changes.

Have a good time!