That's nasty.. its been sitting for 3 months? Definitely not live.. and probably smells terrible.. If it was a completely established sandbed, then i can imagine there was a huge die-off.. and whatever living was in that is dead and rotting now.
I think the best option would be to just get entirely new sand(dry aragonite), and then get a cup of live sand from someones established sandbed. It doesn't take long for the sandbed to become live.. even though it won't be fully live for a little while. Depending on how deep and how much sand you have. Unless you want to thoroughly clean the sand and get a lot of the dead stuff out.