Is your GarageBand version up to date? If not, upgrade to the latest version. also, you coud try to reset GarageBand to the factory settings. That brought my iCloud projects back.
Open the "Settings" app on your iPad and scroll down to the applications. Open the GarageBand settings and set "Reset GarageBand" to on.
BTW: If there are network problems it sometimes may take a very long time, before the song is downloaded from the cloud again. That is why I never upload the original to iCloud - I duplicate the song and share only a copy to iCloud. Once I am sure the upload has been successful and the cloud version is working I delete the original.