Is the key missing from your keyrings? Is it showing in PGP Desktop's My Private Keys? If so, is it Enabled, with Implicit Trust, and non-expired? If it isn't, and you have no backup, you need to generate a new key, and set this as the Default Key in your PGP Messaging account. If all is functioning for you except for that message, it might be prevented if you just delete the key from you Master Key list in PGP Options.