Can't login to WP admin (correct username and password)

I can’t seem to login to the WP admin for our main site (cleanworks.com.au) – when I do it just links to a blank screen without any error messages. I’ve tried clearing my history/cookies and using different browsers and devices but it all yields the same result.

Oddly, I can still login to our blog subdomain (cleanworks.com.au/blog/) which has it’s own separate WP admin – though this isn’t much help as I still can’t modify content on the main site.

Help please!