Assuming it is lined with carboard what I'd probably do is try and fix it with runny superglue.
This is something that has to be done with plenty of ventilation and much care as there will be a lot of fumes and heat generated.
First I'd cover a piece of flat board with brown parcel tape (as superglue won't stick to it) then secure the roof using masking tape, compressing it flat and square afer cutting any difficult joins. The roof would need to be laid out upside down, as it's the inner cardboard that needs access to for fixing into its new position.
The next ingredient is runny superglue available from some hardware stores and poundshops. I use Maxifit which costs me 95p for 20g. I'd then pour the superglue over the inner cardboard, taking care not to flood it.
Sounds like a lost of fuss and bother and has the potential to go horribly wrong but it's how I fix cardboard.