So you have gone to your login URL, most likely this is yourdomain.tld/admin ( you should really change this for security reasons), and have entered your login details as normal.
You are then presented with a blank white page.
This is most likely going to be a plugin that you have recently installed or updated. If you know exactly which plugin you have just installed that's causing the issue your could log in via FTP and try changing the folder name of the plugin. This will trick WP into thinking it has been removed and it will be disabled.
If you haven't got a clue which plugin is causing the issue you try disabling all of plugins in Wordpress via the database.
Once you have disabled all of plugin you can re-enable them one by one and identify the issue.
Another common cause of the white screen is an error in your functions.php. If you have been playing in there then look for erroneous characters, extra lines or duplicate lines. Usually an error like this will cause the white screen on the front of your Wordpress site as well as the admin.
And...one more thing that can also cause this issue is the memory. If your memory is too on the server for the domain you may need to increase it, especially if you have a large number of plugins running.