Actually, when you reinstalled Windows, it may have moved your old data to C:\Windows.old or a similar name. I once did a clean reinstall of Windows and the previous operating system C:\Windows, Users, and Program Files were moved to that folder. Check if you have such a folder. You need to find it fast because Windows is going to delete the whole folder automatically after about a month, and I'm assuming you reinstalled Windows today.
First, reinstall Atomic Wallet, it's going to be a clean state with no wallets imported but it will generate the C:\Users\<user>\AppData\Roaming\atomic. If Atomic Wallet is running then close it. Rename the folder to something else (like atomic2 or atomic3). You can delete this after you copy the folder below and you verify your old wallet is back.
Now navigate inside C:\Windows.old, and go to the path OmegaStarScream wrote:
C:\Windows.old\Users\<user>\AppData\Roaming\atomic
Copy
the entire folder to C:\Users\<user>\AppData\Roaming\ and your old wallet should be there.
Atomic wallet may have also stored stuff in the registry, so if you want to import the registry settings it's using, you can do this:
Copy C:\Windows.old\Users\<user>\NTUSER.DAT and put it in some folder you remember like your Documents folder. This file has all of your registry settings for the current user. If I can find what the registry keys for Atomic wallet are, then you can mount this file on Registry Editor and export the keys to another file, and import them to your current registry at HKEY_CURRENT_USER key.
I don't think this is necessary because I doubt the wallet file is stored in the registry, it should be on the disk.
@OmegaStarScream, do you know if Atomic Wallet makes a folder in AppData\Local too? He might need to copy that along with Roaming.