Double check you installed the right version of the patch for the server you'd like to play then try log in again.
Also there recently was a name purge. If a character was under level 10 and hadn't been played this year it may have been deleted.
If you suspect that to be the case you'll want to create a petition for a character restoration. Make sure you give all the details required (they are probably outlined in a thread in the petition forum) to expediate the process.
Welcome back!
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