The reason can be one of many you need to explain a little more in detail what the problem is.
When you put the key in the ignition do any lights come one?
when you turn the key further does it crank ? Try to start)
it could be that the imobilisor light comes on the key could be faulty you may need to get the key check or re programmed.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix car alarm systems.