The nature of your problem has different meanings and because of this, different solutions. The problem could be related to a bad PSU, motherboard, RAM and videocard. In order to identify the bad component you should first perform a CMOS reset. You can do this by removing all cables from the motherboard and then remove the battery for at least 5 minutes. Plug all cables back (including the battery) and power on the computer. If ii does the same then, try to remove components one by one. In one of those cases, removing a component should give BEEP errors. First you should start the computer without RAM and see if it starts, then, HDD drive and then the videocard. The idea of all these steps is to identify if something got broken. If the computer starts then the problem is identified and you should replace the bad part. Also check the start button and see if it's not locked by any means.
