503 Service Temporarily Unavailable on login

Hi All. Hope you can help.
No users can login to site.

I enter username & password on login screen, click login & then I get a “503 Service Temporarily Unavailable” error.

The front-end site is fine, just cannot get to wp-admin.

I have tried the following…

– Clearing browser cookies & cache.
– Replacing wp-login.php file with a freshly downloaded one.
– Disabling all plugins & current theme by renaming them in ftp client.
– Checked siteurl and home in wp_options table.

Any ideas?

Cheers,
Chris