2000 Cadillac Deville won't Turrn over

My 2000 Cadillac Deville won't start.  This problem has
come from left field. This is the first time this has
happened. My battery is great, lights all work, so I assumed
it was the starter. I replaced the starter, but it still won't
start. I also replaced the starter fuse, and this didn't help. I
had the oil changed two weeks ago, and my oil looks great.
My security light has been on since I bought the car last
year, so I really don't think that is it. I don't know. A my lights
on the dashboard do show up, I do have power, it just won't
crank. Its comete silence when I turn my key over. If
anybody can help, I would greatly appreciate it. Thank you
in advance.