Hi,
you need some data management on top of your actual logic in game for collectables.
each collectables should have a unique ID, and in your data ( player prefs most likely), you will save the current state of that ID, likely a simple flag saying it's been picked up.
when you are about to instantiate a collectable with a given ID, you must first check against your data if you should show it or not, that's key when you load a level or restart a level.
Bye,
Jean