First step to check whether its a hard ware or software issue .........disconnect all the external devices and disconnect the power cable from the back of the computer , then press and hold the power button for 10-12 secs with the power cable disconnected and then reconncet only the power cable and restart the computer and check whether it restarts or not.................shut down the computer and while u restart the computer keep on tapping f8 key on the keyboard and on advance boot options select last known good configuration and hit enter and see whether it restarts or not .......................after that shut down the computer and while u restart the computer keep on tapping f9 key on the keyboard and do the diagonistic test and see wat error do u get if u get any biohd-8 then the hard has gone bad , if u get any other error then jast open the tower and reseat the hard drive and the sata cables , the hard drive cable will be jst below the dc/dvd driive looks lik a bunch of multi colured cables .............and also reseat the sata cables i.e a red ribbion like cables and restart the computer and see ....