JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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I think your problem is related to encryption, if you can provide us with some details about the specific issue with private key, we should be able to help you.
Note that, if you lost your private key, we cannot recover it as it is not saved in JotForm server.
The best thing to do if you lost the private key, is disable encryption for every form and do not try to create a new key since it will not be able decrypt the previously encrypted submissions.