If it's the white screen, that's normally a PHP problem, so I'd guess a theme or plugin issue.
therefore start troubleshooting by :
-deactivating ALL plugins temporarily to narrow down the problem. If the problem goes away, activate them individually (one-by-one) to find the problematic plugin.
If you don't have access to your admin area, try deactivating plugins manually via FTP or phpMyAdmin.
-switching to the unedited default theme (that came with your version of WP) for a moment by renaming your current theme's folder in wp-content/themes. The idea is to force WordPress to fall back to the default theme to rule out any theme-specific issue.
If you don't have access to your admin area, then access your server via FTP or SFTP or whatever file management application your host provides.
- resetting the plugins folder by FTP or phpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive).