If your in the App Store and Garageband shows a price for purchase, either:
1. You are not using the Apple ID that you used originally on your Mac when you first purchased it. If your Mac had Garageband pre-installed on this Mac, that Apple ID you used when first getting the Mac should be associated with your pre-installed Garageband copy in the App Store. Like Leonie (the zen master says) , having the correct Apple ID signed into the App Store and clicking on the Purchased tab at the top of the App Store window would show you the Install option, if it's somehow been deleted/uninstalled/erased off the Mac.
2. Doesn't have his Apple ID signed into that App Store at all.
3. Got this Mac second had, where his Apple ID never had a purchased copy of Garageband.
If you did purchase this second hand and it was on the Mac, what ever caused it to be "suddenly gone" now is basically telling you, you will have to purchase a copy.