What likely happened is after logging in you lost the model packs because they were never tied to your account.

If you can get in touch with the original owner there is an option to "gift" the licenses to your account.

http://line6.com/support/docs/DOC-4738

(from linked doc)

Q: What if I sell my device? What happens to my licenses?

A: If you intended to sell your licenses with the device, then you can “gift” the relevant store order(s) to the buyer (see how to “gift” below).

If you didn’t intend to sell your licenses with the device, when the buyer runs the License Manager and logs in, it will prompt him to authorize the hardware, which will remove the device authorization from your account and restore the licenses for use with your account.

Q: If I buy a used unit with packs installed, can I transfer them to another device?
A: If you are a buyer and have purchased a Line 6 device with model packs installed, since you did not originally purchase your model pack, you do not have the license key in your purchase history. The owner would have to “gift” the model pack’s store order to your account (see how to “gift” below). You'd then be able to use the model pack on any of your authorized devices.

Q: Can I transfer model packs from my account to a different account?

A: Yes. This is called "gifting" (see how to “gift” below).

Q: If I sell/return my device, do I lose my packs?

A: If you intended to sell/return your licenses with the device, then you can “gift” the relevant store order(s) to the buyer (see how to “gift” below).

If you didn’t intend to sell/return your licenses with the device, when the buyer runs the License Manager and logs in, it will prompt him to authorize the hardware, which will remove the device authorization from your account and restore the licenses for use with your account.

Q: How do I “gift” a store order?

A: Log in to your Line 6 account, visit the Purchase History page, and click the Details link for the order.  Click the link/button labeled "gift", then enter the destination Line 6 account username and click confirm.  The order will still show in your account history, but you will not have access to the license(s) from the order, be able to return it, or be able to gift it again.  In the giftee's account, they will see the order in their history, but not be able to return or gift it.  They will have access to all the license(s) in the order.