Its possible your old account was hacked / used by RMT, and they would have surely changed your password. Not saying this is the case but its possible.
Also, as other people have said, you have to make a SE account from your POL ID, which can be done
here. Before you do that though just make sure your account wasnt hacked. You may have to call SE to see if it was.
If it was hacked.....its not too bad starting all over. Your new registration codes you just bought will support a new account anyway, and levelling to where you were back in the day will probably only take you a month or two at most.
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Manifest, Valefor
BST/PLD/COR/BRD/WAR/THF/SMN/BLM/WHM 99
RDM/SCH/DNC/NIN/WAR/SAM/RNG 49
PUP 30, BLU 16
Maniken, Valefor
WHM/DRG/BRD/MNK/RDM/WAR/BLM 99
SCH/NIN/SAM/BST 49
BLU 16
Linkshell: ChaoticUnion
Sozu Rogberry(pre Thf Knife patch): 0/49 ><