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2002 Nissan Altima. Anti-theft problem?

I had a new computer put in this car, and one of my two keys was “flashed” to match the computer. After a few weeks, my car won’t start, and I wonder if I accidentally used the wrong key and some sort of anti-theft or something is keeping it from working. I have now tried both keys with no success, so I don’t know if I’ve really messed it up, or if it has nothing to do with the keys. Any ideas on how I can narrow it down? Thanks.