wideblueyonder’s solution worked for me. Both my website and admin area was blank.
1) Downloaded newest 3.1.
2) Deleted wp-admin and wp-include directories and replaced all files with those from the download. Also replaced all other files except for those in wp-content.
3) For wp-content, I was more selective and manually overwrote old files.
4) The admin area then asked me for database details again because wp-config.php was not found. I entered my DB details again (thankfully I remembered them). When I got to the install portion, I closed all browser windows and went back to the admin area. It then asked to upgrade the DB, so I did.
All working ok now finally. Phew.