Unless they are specifically multiplier onions, onions are an annual plant if you want to harvest them for food, or biennial if you're saving seed. You put sets or seedlings into the ground in early spring and in late summer they flop over, the leaves start turning brown and they are ready to harvest. You shouldn't have left them in the ground until late fall and certainly not over the winter.

The ones you left in the ground will probably produce seed stalks this year, but they won't make edible onions.