Have you opened the vent at all?
How warm was the temperature when you left it in the sun?
If it delaminated due to internal pressure, you would see fairly large areas that are curved/bubbled (blown up
away from the core foam) relative to the surrounding areas of the board.
The fin box is a very thick carbon fiber reinforced part, that is set into the board and bonded to the foam core and to both the upper and lower skins of the board.
It's very doubtful that you could build up enough pressure to collapse the fin box, even if there was
enough internal pressure to delam the skin from the core foam.
If the temperature was not more than say 85 deg. F ( 29.4 deg. C) there is very little likelyhood that you
delaminated the skin from the core.
Are you sure that your fin fit nicely before this incident.
Try a bar of soap (bath soap) rubbed all over the fin head (the part that goes in the fin box in your board)
and it may slide in much more easily.
Normally, you have to leave your board locked inside a closed vehicle where the temps go well above 120 deg. F ( 49 deg. C) to get a modern sandwich board to delaminate.
Hope this helps,