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This has happened to me. Firstly at Santa Pod and a very kind guy helped me out with a WD40 type spray and that solved it for a few months until I was at an expensive private trackday at Nurburgring. First lap.. Came in to garage (F1 pitlane).. Opened bonnet to check and cool.. 10 minutes later I couldn't close it. Fortunately Hurdy helped me by waggling around a screwdriver for a good few minutes. We checked fluids first and then said "Right! This bonnet doesn't get opened again until we're back in England!"
I went to my VW dealer when back home and he told me it was fairly common on Mk5's and they had done one the previous week and now continually stocked the replacement parts. Needless to say, having a carbon bonnet I decided to fit some AeroCatches as insurance < A great excuse!
First step is to lubricate the cable mechanism.
If it happens again you'll probably find it's a failed lower locking 'block' (spring mech failure/corrosion) which needs the whole lower unit replaced (like mine did).
HTH