I received an encrypted submission and can't read it

That's because the form encryption is enabled in your form when you received that submission. You can only decrypt it when you enable the form encryption in your form again so when you visit your form submissions page, you will be prompted to upload your encryption key (private key file) before you could see the decrypted data.

To learn more about how Form Encryption works, please check our guides here https://www.jotform.com/help/344-Encrypted-Forms-and-How-to-Use-Them

I enabled encryption and uploaded the key that was created previously and nothing happened.  It's still encrypted.

You have to use the first private key file that you've generated initially when you enable form encryption. If you have created a new key, it would only work for the new submissions, not the existing ones. If you can't find that key, I'm afraid there's nothing we can do since only you who has access to your key.