Something went wrong when you exported the .fbx from whatever program you were using. This happens to me specifically when I use blender and I forget to make sure I have an object selected but I specify in the exporter options to export only selected items. It creates an empty fbx file. Right click on the file in windows explorer and choose properties to see how big the file is. It will be very small, maybe around 4kb. Even a simple cube will be above 10kb. This might help you track down what went wrong. Let me know if you need a little more help.
I think this might be the same person as before.
your folder-structure is too long.
you cannot have more than 256 letters in your folder structure, so place your content closer to the base of your harddrive.