Sorry about your problem, I know you hated to have that happen! In my experience, the trailer doesn't have to be perfectly level to lower properly. I would suspect some binding....somehow, but am sure you examined the broken panel for anything that might have caught on the frame (especially the sides of the panel).
You might get a replacement from J&R Repair Home - JR REPAIR
but I would think you can make one for less than the cost of shipping.
You'll get everything sorted out soon!