S3 stuck on first splash screen I recently installed an update to my custom ROM (http://forum.xda-developers.com/galaxy-s3/development/wip-cyanogenmod-12-t2936990) which was previously working fine on my phone. After installing this update my phone now sits on the "Samsung Galaxy S III" screen and after about half a minute reboots back onto the screen.

I can access recovery and download mode with no problems still and I've tried re-flashing recovery and the ROM with no success.

If anybody can suggest anything I'd greatly appreciate it, thanks.

EDIT: I am unable to actually turn the phone off without removing the battery, every time I hold the power button it just turns itself back on after a second.
Have you tried to do a data wipe/factory reset?

You could take a nandroid backup, and use an app such as Titanium backup or Nandroid manager to get (some of) your data back.