In case anyone else lands on this thread from 10 years ago and finds this helpful:
Since you said this form is quite long and contains several field sets, it may just be having trouble loading. I would advise you to simply wait a little longer and see if the form loads. Our forms take several seconds to load and appear like your screenshot because they too contain many fields and sets. My solution may seem irrelevant, but I don't know how complex this form is and it may truly be an issue of waiting.
Hi @Vlada_Prasolova, I have faced the same issue 2 years ago when I added the jquery in HTML code and code shows syntax error. To fix this, I have tried to clone the form. Luckily when I cloned the form, editor opened so I deleted the code and everything works fine.
IF there is no code then try to clone the form.
Might be cloning of form will save your time.