"wp-admin" is blank page

I don’t know what has happened but for some reason my /wp-admin just takes me to a blank screen.

I am already logged in in firefox though, but whenever i make any changes it takes me again to a blank screen.

I haven’t edited anything except my current theme but I don’t see how that could be causing it? And i’ve only really modified the header and even restored it…

The problem seems to go away if I use the default theme.

Any clues on what could be causing this?