laptop fails to start up

well i use my dell laptop latitude D600 almost for two year now.
but three days ago, i was use it..then it stack for five minutes then,i decided to shutdown it manually by press and hold a power button. I turn it on, then its wont start,i check the hard disk,RAM,CD-ROM,battry and power adapter all of them are fine. so what should i do to handle this problem of my laptop? its fails to start
!?