Asus Zenpad Tablet fails to boot

My Asus tablet has decided to hang on bootup.
It gets to the splash screen and the flashing circle continues to rotate, it goes no further.

Tried PWR & Vol Up - which gets to a fastboot loader, but cant do anything else apart from hold pwr button until it shuts down.
Also done PWR & Vol Down but only a black screen appears.

Any ideas?

Thanks