You mean you forgot to save your wyBuild project file? If that's the case, and you can't recover it, then you need to distribute new client.wyc files to your users before they can update. The reason this is the case is that wyBuild uses signed updates (see: Signing Updates in wyBuild).
In other words, your new updates are signed with a key that's different than the key of your old wyBuild project. Thus, wyUpdate is rejecting the update.
Does this make sense?