Be aware that storing a GarageBand song on iCloud from inside the GarageBand app and backing up your iPad locally or on iCloud are two different things.
If any song was stored only on the iPad and wasn't pushed to the iCloud, then I guess it was deleted with the app. If you don't have an older iPad backup (locally or iCloud) to restore that app, then I guess you are out of luck.
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'