How to repair damp under roof and has expanded after heavy frost?

I have a bessacarr camaeo 420 sle 2 berth touring caravan. when it rained water was coming in through the bathroom light fitting, this was allowing it to bleed into the roof under lay, we then had heavy frost and the water froze and has caused a bulge in the roof, can any one advise me if this can be repaired.