Garageband would not open today!

Hi

Have recently installed the new Garageband (after a clean install with Mavericks) on my Mac and my youngest son has been using it.

He has saved several projects and has tried to open GB this morning. He can not open any of his recent or saved projects

nor start a new one. Have deleted GB for him using AppZapper and restarted my Mac.

Have now tried to download it again from the App Store, but it says it is installed on my Mac.

Any help gratefully appreciated.

Thanks

Matt