Welcome at Dominion Online, MarieBrutale!
In theory you should have gotten an email at or around end of 2016/start of 2017 to manage the 'transition' of your old Goko account to this (then new) place. I guess that email, even if you received it, is long since gone (maybe you can take a look if you/your provider keeps copies).
Players from Goko would get the expansions they had on Goko for one year here, then they would have to pay. As an explanation, the second edition base set is free for everybody, but expansions here are 'rented' on 2 possible levels (Silver, roughly half the expansions; and Gold, which includes all currently existing expansions) for a given time (one month on Silver is approx 2.10EUR, while one month of Gold is approx 3.45EUR).
Now I *think* that that 'free year' was supposed to be during 2017, but I'll ask if it can be started now. Dominion Online got the Dominion user database from Goko, so they can easily verify your claim if you provide username and still have access to the email adress you used for that service.
ADDITION: I asked, and unfortunately the 'free year' was only meant for 2017.
I understand that loosing access to the cards you thought you bought is frustrating and unsatisfactory. But the main reason for this is that Goko (the previous rights holders for an online implementation of Dominion) did not have those rights for an unlimited time, and the rights contract was not renewed.