Bird Box Attack

 Hello everyone!

it’s my first time posting on here and a newbie to rspb. We moved house in October in to location that attracts more wildlife. We’ve got two bird boxes on the side of our extension that the previous owners had up. We think blue tits might have been nesting in them. 
This morning I opened the curtains and found 5 dead babies and two part hatched eggs on our decking, fallen from the bird box. When I looked at the bird box the front in missing (see image) 

A couple of questions;

1. Is it safe to take down the box or best to leave in case there are survivors?
2. does anyone have any idea of what could have done this?

3. When is it best to do bird box maintainence?

thank you in advance!

we’re heartbroken for the seven babies that haven’t made it.