first of all
open ur PC cab
n than start ur PC, see that ur processors fan is running or not if yes than check that may be their is some loose connection
n pls write in detail that wht do u have in ur PC
may be ur lan card is been shorted (try removing that), or may be one of ur ram
if Power led & HDD led is on that means no prob with soft & ur mobo & harddisk is also fine