Can't login in via domainname.com/wp-admin

So today I had a client brick their WordPress website by updating their WordPress core. There were no recent backups.

Ironically I was in the process of adding a backup system for some other client websites just before this happened…

Anyway, I’ve managed to put the site back up using a very old version I had stored locally. The website displays correctly, but all of a sudden I can’t log in (or out – I’m stuck in the WP Dashboard with Chrome and locked out with Safari).

Has anyone experienced this? I’ve tried putting the plugins folder on hold and troubleshooting that way. I’ve also tried reverting the version of WordPress using the backup but I fear that the database itself has been adjusted via the WordPress updating process and this could be my issue.

Any help would be absolutely appreciated!

Cheers,
William