nissan almera 2000 phase 2?

in short, my car needed a new battery, so, i removed old one and put new 1 in, turned key.........
guess what.........?
was turning over but wouldnt start.
phoned nissan who said it keys "mite" have dropped there code,
dont think so as its not remote keys, so no battery or anything in them, plus never touched the keys was the cars battery i changed. hopefully there a easy and quick fix for this. need my car.
and it aint goin to nissan delaer.

thanks in advance