Hi,
I have this happen fairly regularly ... whenever I introduce an exception in my initialise or calculate events (could be other events as well).
Try opening Acrobat first and the open the temporary file Designer uses to preview the form, which is created in a folder like, C:\Users\<<userid>>\AppData\Local\Temp, and will have a random looking name like, "_e5k26d0p3eeb6mp.pdf".
You do have to open Acrobat first, you can't just double click on the temporary file.
Seems this way you will get the exception reported in the JavaScript debugger console.
Good luck
Bruce