Well, according to section 10.1.15 of the Enigmail handbook:
10.1.15. I lost my passphrase / my key pair / my private key.
A note: Your private key is bundled with your public key in your key pair, hence
losing your private key and losing your key pair means exactly the same.
There is no way to recover your passphrase: your only hope is to try to
remember what it was. If you don't succeed, you lose the use of your private
89key, and hence your whole key pair is now useless. There is no way to recover your private key, either. It cannot be obtained from your public key or from any message that was signed/encrypted by that private key. You can only recover it if you made a backup in the past.
Hence, losing the passphrase or the key is definitive. If you generated a
revocation certificate (and you should have), use it to revoke the key pair.
You must also generate a new key pair, send the new public key to your contacts
and warn them not to use the old public key any more.
Messages that were sent to you encrypted with the old key cannot be decrypted
any more. Messages that were signed by you with the old key can still be
verified by the recipients by using the old (revoked) key.
To avoid this disaster, it is recommended that you backup in advance your key
pair: from Key Management, select File → Export Keys to File, make sure you
included the secret key, then store the file in a safe place. Make sure you
chose a passphrase you can remember, too.
Unless going to Key Management does the trick.
If you did export the keys, you could try doing it again and check what file format they are being exported as and do a long, very long search on you C drive for that specific file type and see what and where can be found.