The chassis is locked by the seat or bassinet unit clicking down onto the black prongs that stick up from the central hubs. It would be mechanically impossible for the front wheel assembly to swing back if the seat was fully engaged into the hub as it has a large plastic block that fits into the hubs to lock them. Remove the seat check for any obvious damage, spray some silicone spray on the prongs and the square white lock buttons. Wipe of excess with a paper towel than try to fit seat again.
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