500 Server Error when calling wp-login.php

Hi all,
I’ve tried all the ‘fixes’ I can find on the internet for this one and getting nowhere fast. Hopefully some kind soul can shine a light on the problem.

The website appears to be running fine, but this morning when I tried to log-in to the dashboard I get 500 Server Error. Since then I’ve renamed .htaccess, ‘plugins’ and ‘themes’ and manualy loaded a fresh installation of wordpress (deleted & installed wp-admin/wp-includes and all loose files) … nothing has worked.

Can anyone help, please?

Thanks
KarlB