If the GarageBand app won't open after updating your iPhone, iPad, or iPod touch to iOS 11, Apple says to turn off iCloud for GarageBand on the device in Settings > Apple ID, iCloud, iTunes & App Store > iCloud.
After toggling off the feature, you won't be able to access any GarageBand songs stored in iCloud Drive until this issue is fixed, but they won't be removed from iCloud. New songs will be saved to your device locally.
Several users have reported the app crashing upon being opened on Twitter and the MacRumors discussion forums, but it doesn't affect everyone. MacRumors hasn't been able to reproduce the problem on multiple devices.
GarageBand is free on the App Store [Direct Link] for iPhone, iPad, and iPod touch.