I don't have ubuntu in front of me at this moment, so the help I can offer is limited, but you will almost certainly have to generate a new key. You should be able to do this from the passwords and keys dialog.
I copied those instructions from here: https://help.launchpad.net/YourAccou...tingYourPGPKey
Step 1: Open Passwords and Encryption Keys.
Step 2: Select File > New, select PGP Key and then follow the on-screen instructions.
Now you'll see your new key listed in the Passwords and Encryption Keys tool.
You may have already seen that.
Also useful is this page: https://help.ubuntu.com/community/GnuPrivacyGuardHowto
There is a section on how to generate a pgp key, with a variety of methods listed, just in case you have any trouble with the passwords and keys manager.