It's highly possible that you've logged in to a different account, which isn't linked to your channel. In this case, you will see the channel in your creator studio if you sign in Castbox using the same way that you've signed in before.
Signing in Castbox using different ways will create different accounts. If you cannot find a channel that you've claimed or created before, please check your account first to make sure that you log in with the same method.
Suppose that your channel is under the account that you sign in with Google, the channel won't show up in the creator studio if you register and sign in an email account using the same email address from Gmail.