I tried to run a scheduled WP update on my site (which has been sat dormant for a few months so hasn't been updated for a while) yesterday, and since haven't been able to log into my admin/dashboard area at all. The site is still live and working fine, but there seems to be no way of getting to the admin area. It doesn't matter whether I can log in via wp-admin or wp-login.php. I can get to the password screen, but every time it redirects me to wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F which then leads to the following error (Not Found. The requested URL /wp-admin/upgrade.php was not found on this server. Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request) or a blank white screen. When i ran the update it also resulted in a blank white screen. I have tried everything in the codex as well as quite a few 'fixes' I found online but nothing seems to be working.
I don't actually mind losing posts - as the site is currently live I can copy the existing posts and repost them and there aren't many anyway, but my theme is heavily customised and I don't want to have to recreate it. Is it possible to reinstall a clean version of wordpress but somehow restore my theme customisation? I don't have a recent backup (duh I know!!) so can't restore from that. I'm running out of ideas.