Your PC starting up comes in 2 parts. Posting and booting. Posting is the black screen part that runs through the hardware parts, booting is the OS starting up. Now based on your description, the PC isn't posting. Which may mean you didn't put it back together right. Crack open the case and try and turn it on again. See if power is getting to CPU fan and the MB. If it is then check the RAM. See if it's inserted all the way. If you can get all the connections right and it still won't post, either a RAM stick or the CPU is dead. A PC should still post even if the HDD or video card aren't working.