Have you checked carefully about the "not dead" part?
Okay, it may be that you didn't vote in enough previous elections to keep your registration current- or most likely that the election board screwed up. Did you vote using a contested ballot? If so, the election officials can look you up in the master system, and then record your ballot (typically this is done immediately unless there are mass amounts of contested ballots- but that is a different thread).
I know in our precinct the "little old ladies" (God Bless them) are the election judges. They don't always see well, and get confused about things like hyphenated names (Mine is hyphenated and they kept telling me I wasn't listed under the second name....had to show them that I was listed under the first- as it was written on my voter registration card). There are millions of names, and statistically likely that a few will get messed up- No problem, unless it is YOURS!
Good luck. And thanks for exercising your right to Vote! This is an historic election, regardless of who wins.....and we Americans seem to be appropriately rising to the occassion.