No air in one room in flat roof house, why?
Either a damper is wrongly in the closed position or ducting has come loose. If it is an adjustable damper, then it should in an accessible location, e.g., in an attic utility space. Another possibility is that a duct is plugged or crushed.  

If you want to try to analyze the problem yourself, remove the register grill in the room and see what you can. If the duct makes a turn near the register, then you won't be able to see far. You could lay a flashlight inside, then use a mirror on a rod to look inside the ductwork.

I would think that there are cameras that are used to look inside HVAC ductwork, but that would be a professional instrument. Plumbers use those to examine sewage drain lines and you'd want to be sure if you got the use of a camera that it had never been used on drains.