Well thre could be several causes. If in case nothing boots up that means the sytems dead, try checking all internal conections are in place (cables,battery(witch stores CMOS info(if this one fell out, you might as well consider bying a new motherboard or reinstalling and reconfiguring thw whole BIOS system, witch isnt exactly easy))etc.) Also check your graphic card is in corectly, because the computer might be actually booting up, but somewhere between the motherboard and the monitor is a faulty conection.If you do boot up, defrag your drive, clean all unecessary files,folders etc.. Make sure you have more than 15% free space on hard drive. Run some tests on ram and the drive. Because you might have some faulty hardware. If the problem is not hardware, then propably either somewhere bad conection or some wrongly installed configured software. If you installed something new or you acquired a virus , its propably the cause. Email my friend , he gave me this info, hes real comp pro.
GL