take it back to where you got it from (if a dealer)- the flap is connected via a cable ( the release handle/tab is by the drivers right leg)
so not to much can go wrong tbh 1. the cable has gone stiff and needs oiling or replacingor 2. the petrol flap has been forced/broken
.....had a similar experience with a Mk3 Cortina. My mate was helping me fit some uprated springs and adjustable dampers on the front, but the bolts had seized in the damper mountings. So we jacked the engine up, to drop out the cross member/subframe, to drill out the bolts.
Dragged the cross member into the garage, and my mate dropped the bonnet as it started to rain. The result was a neat power bulge.
My 21 yearold Honda's bonnet was quite heavy and when lowered would shut with a realclunk. I seem to remember during its lifetime some cars fitted with earlyairbags would 'fire off' when the bonnet was dropped shut! Any one here remember that episode?