You now have a choice - it is recommended that dahlias are dug up, the topgrowth cut off (if there's any left) the tubers cleaned of soil, and left upside down to dry out on a tray of sand, then after a week or so, potted up into sand and peat or sand, kept somewhere frost free over winter, then repotted early next Spring to start them into growth, inside, and planted out when all risk of frost is past (end of May). Lots of people, though, leave their dahlias in the ground, and sometimes they survive. If you have heavy soil and live in a cold or very wet area, they're less likely to survive in the ground, but they don't always survive even if you take the recommended action.