asters are hardy and should come up from the roots. however asters are late flowering plants. Maybe they haven't just poked up yet. or perhaps they may have suffered during winter due to an air pocket in the soil. Air pockets can kill plants in winter.

Asters actually do better planted in the spring, so their roots have a chance to get established before winter sets in, if you can find them. Fall planting is chancy if you want them to live over. I always plant mine a bit deep so they will take root around the crown, just like I do with mums, but I still lose about 2 out of 5. I always buy a bunch on sale at the last of the fall season so I don't have a lot invested.