What is my secure key to access encrypted form?

I dont remember that I created one! 

Please note that when you enable encryption in your form, a key file is downloaded. Have you saved this key file in your computer? To decrypt the submission data, you need to provide the same key file.

If you are using the same computer which was used to enable encryption, the key file will most likely will still be there in your downloads folder. You may like to take a look at the following guide which should help you:  https://www.jotform.com/help/344-Encrypted-Forms-and-How-to-Use-Them#Where-Can-I-Find-the-Private-Key-After-Downloading-It 

If you have lost the key file, I would suggest you to please disable encryption in your form to ensure that new submission data are not encrypted.