My form is not appearing. Why is my form blank?

https://form.jotform.us/Francesco_REP/optimism-test

I created a form in January and I've used it several times. I've updated it and it worked yesterday. I have a new submission. However, when I try to access the form now. The screen appears blank. 

I checked your form and it looks like all the fields have been removed.  I am not sure if they were deleted on your end or a bug on our end, but either way you can revert your form back to the way it was using its revision history:

https://www.jotform.com/help/294-How-to-View-Form-Revision-History

That is very odd because when I edit the form, all the fields are still present and even the fields with calculations. Nonetheless, thank you for the instructions to revert to an older version. I've reverted and the form is functional again. Could this be a bug from having created the form in Version 3 and editing later in Version 4?

Thanks for your help!

There should be no issue with switching between editing the form in version 3 and version 4.  You should be able to switch between them without impacting the form.