Garageband is stuck on this page and stops responding

If it’s stays on that screen, it should say what plug-ins it trying to validate. You could try and remove or disable the non validated plug in.

If your not comfortable doing this by self, call Apple Tech support at 1-800-MY-APPLE, they will help over the phone to resolve.

Option + Command + esc to bring up the Force Quit panel if you're wanting to shut it down.

Or press and hold the power button until the screen goes black ... about 10 seconds.

Have you tried reinstalling GBand?