Laptop won't boot

I have a Dell latitude laptop which has suddenly stopped working. When I press the power button the led lights and the fans run but the screen displays nothing.  After about 15 seconds the power light goes off but the other side leds (eg for the wifi, battery)  come on, almost like it has gone to sleep. I've tried charging the battery  and running it from the ac without the battery and that doesn't change anything.  Can anyone help?