If you are not a United States citizen and you registered to vote, you could face criminal charges, denial of immigration benefits and even deportation from the country. These possibilities get even worse if you actually voted in an election.
There are federal and state laws that prohibit people who are not eligible to vote (usually non-citizen immigrants) from registering to vote and voting. It is a federal crime for a non-citizen immigrant to vote in a federal election. If you are not a citizen, you can be prosecuted, sent to prison and deported for voting in a federal election, even if you did not know you were not allowed to vote.
The very act of registering to vote requires that a person certify that they are a United States citizen and eligible to vote. Anyone who registers to vote and is not a citizen, has likely made what is known as a false claim to citizenship. A false claim to citizenship can stick with a non-citizen forever and stop someone from getting a green card, getting citizenship and can even lead to detention an deportation from the United States.