If you registered an account at
https://en.nostalrius.org/#iwannaplay , then that is all you technically need. A forum account (
https://forum.nostalrius.org) is not needed unless you feel like participating in discussions with the community.
Your game account and forum account are not linked; the two are completely separate. If you change your password under your profile here, it has no bearing on your game account. Earlier when you said "you even changed the password," you only changed it here on the forums; your game password is still whatever it was when you originally registered.
If you're getting the "this account has been closed" message, you're likely logging in too many times unsuccessfully and getting temporarily banned by the server. The ban expires after roughly 15 minutes and then you can try again. Otherwise, if you don't remember what your password was, you can reset on the website, or here:
https://en.nostalrius.org/password