Has it had aeration and water flow during those 3 weeks? If not, then most, if not all of the life that was in the sand is most likely dead. Not something I would add back to my system untill I could rinse it with some old tank water during a water change. Once it appears (and smells) clean, it could be added to your tank/sump and seeded again with another pound or two of live sand from either a different tank or purchased. If you are looking for the sand to simply be a substrate for bacteria to grow upon, then simply rinsing with old tank water would suffice.
Chuck