This article is from the PGP FAQ, by Jeff Licquia email@example.com with numerous contributions by others.
Assuming that you selected a good solid random pass phrase to encrypt
your secret key ring, you are probably still safe. It takes two parts
to decrypt a message, the secret key ring, and its pass phrase.
Assuming you have a backup copy of your secret key ring, you should
generate a key revocation certificate and upload the revocation to one
of the public key servers. Prior to uploading the revocation
certificate, you might add a new ID to the old key that tells what
your new key ID will be. If you don't have a backup copy of your
secret key ring, then it will be impossible to create a revocation
certificate under the present version of PGP. This is another good
reason for keeping a backup copy of your secret key ring.