I recently had this same problem. Deactivated all the plugins. Deleted the necessary files. uploaded all the files – the blog was showing up fine – but going to wp-admin showed a blank page. could not hit upgrade.php (just a blank page). So I deleted the new files – uploaded the OLD ones – then I had no problem getting in to wp-admin. Tried it again – deleted the old files, uploaded the new ones – again, a blank screen. The most frustrating part about it is that I called the host provider and they said they weren’t seeing any errors come up – so no clue where to begin to look for a the problem.