Update to itunes 12.3.1 doesn't work. Blank white screen

Hi,

Ive just updated to iTunes 12.3.1 and iTunes now doesn't run. I just get a blank white screen with a white box in and no iTunes top menu.

Ive tried reinstalling, which doesn't help. Also tried to restore previous version from time machine, but get a message saying changes to iTunes aren't allowed.

I'm running OSX 10.8.5 on an early 2011 Macbook Pro.

Any suggestions welcome.

Thanks