Error when upgrading to latest WordPress

Most likely your theme or a plugin is causing this issue.

Try manually disabling plugins:
https://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/

If that doesn’t help, try disabling your theme by renaming your theme’s folder. It will automatically disable it.

I don’t have any themes running at all, and I tried renaming the folder, but still getting the same error.
Could it be anything else?

It’s not possible to run WordPress without a theme. Did you do a extended upgrade (looks like you are running 3.5.1) ?

https://codex.www.ads-software.com/Upgrading_WordPress_Extended#Upgrading_Across_Multiple_Versions

Do you have a backup of your site from before you tried to update?

It’s not possible to run WordPress without a theme.

Or do you mean that there are no themes in your themes folder?

Sorry I meant plugins!

…I don’t have any plugins and I renamed my themes folder.

Okay, so what version were you running – you selected 3.5.1 for your version when you posted?

Yes that’s the version I’m using at the moment.

Can I send you my website log in so you can have a look?

You can post it to a pastebin – see:

https://codex.www.ads-software.com/Forum_Welcome#Posting_Code

But as I asked above – did you upgrade per:

https://codex.www.ads-software.com/Upgrading_WordPress_Extended#Upgrading_Across_Multiple_Versions

Do you (or your host) have a backup from before you did anything?

How do I go about updating to a lower WordPress…
I’ve downloaded the WordPress 3.6 zip, so how do I upgrade via my Admin as I don’t know how to do a manual update?

Read this:

https://codex.www.ads-software.com/Upgrading_WordPress_Extended#Upgrading_Across_Multiple_Versions