White Page / Screen of Death

So this happened a couple of hours ago to my website.
I am not the developer of the site, but am fairly capable and can code simple things with very clear instructions(!).
I have tried to fix the problem by doing most things that have been suggested on google. I can still access my admin pages.

I have reinstalled the whole site (except the content as this is a pretty big file).
I have updated the WP theme a couple of times to make sure: I have my own theme so maybe this doesn’t make a difference?
I deleted the theme folder via ftp to see if this worked (I read that if you do this it goes to the default theme).
No joy so I reinstalled the theme.
I deleted all unused plugins and deactivated all active plugins.
I tried to check for error by adding code to the wp-config.php – don’t think I did this correctly so reinstated wp-config.php.

This is where I’m up to. If anyone has any clues I would be veeeerrrryyy grateful!
Thanks