Many voter registrations are canceled between elections, often for frivolous reasons. If you moved you'll need to update your address. Your name may have gotten confused with someone else's. There may be a trivial discrepancy between the voter rolls and information in public records, like a typo. Or a clerk may have gotten excessively zealous about purging voter rolls. Some people just do not want you to vote.
So check and check again that your registration is valid.
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@DavidSalo
Also, signatures not matching.
Mine changed since I originally registered to vote. I had a mail in ballot challenged by Norwalk with my signature.