Blank screens are due to any one or a combination of 3 problems.
1) Blank spaces in your wp-config.php, functions.php or a combination of both - open the files and take a look, rectify if needed and try again.
2) Upgrading without deactivating the plugins interferes and botches the upgrade - use your ftp and rename the plugins folder, creat an empty folder with the same name and try again.
3) Upgrading from earlier version having skipped a version or two botches up the upgrade. Files usually affected are those in the wp-admin and wp-includes - delete them and upload new ones from the same version as is currently installed.
Hopefully this should solve the problem and give you access to your blog.