A white page like that means that something caused an error – so you need to manually deactivate your plugins and possibly your theme as well. Use FTP to access your files on the server (ask your host if you don’t know how to do that), find your wp-content folder and inside that rename the plugins folder. See if that lets you login and your site to come back up. If not, find your themes folder and inside that find your active theme – rename that one. That will force WP to use a default theme and you should be able to get back into your site.