lost private key

Here is a snippet from the help article Encrypted-Forms-and-How-to-Use-Them which I highly recommend you read.

Q13: What Will Happen If I Lose the Private Key?

If you have lost the private key, then there is not much that you can do. It means that your encrypted data is lost forever, there is no copy of the same on our servers and it is not possible to crack the one that you had.

Our only recommendation is to turn off encryption right away so you can start receiving submissions normally again. Leave the encrypted submission data on your Submissions Page just in case you find the key at some later point in time.

Note that if you can see decrypted data in some browser, but the key is lost, it may be possible to restore the key from that browser's local storage. If this is the case, contact our support for instructions.

Q14: What Should I Do If I Lost the Key but Wanted to Keep Encryption Enabled?

It is possible to generate new keys by disabling the encryption feature and then enabling it again. Use the CREATE ENCRYPTION KEYS FOR ME option to generate a new key.

In case you're wondering, no, the new keys will not decrypt the old data.

The private key is usually saved in the Downloads folder of your computer. If you have lost it, the submission data cannot be retrieved.