Does your Monitor show anything (like a VGA or DVI box or any other writing) moving around the screen? If you see a box like this if your monitor is OK. If you don't see a Box or writing the monitor might be the problem.

Can you hear the Hard Drive spinning? Put your ear up close to the Hard Drive to see if you can hear it spinning (make sure to have the computer turned on) (don't touch the hard drive)!

Is the Fan on Top of the Heatsink spinning? If not make sure it's plugged in to the motherboard s 3 pin connection marked for the heatsink fan. If it's plugged in, try another 3 pin connection on the motherboard!

If these three steps don't help, I'd go to the 15 steps below!

1. Unplugging everything including the Heatsink and CPU
2. Put CPU back into the CPU Socket
3. Apply small thin layer of Thermal Grease to the center of the CPU
4. Put Heatsink on top of the CPU and secure the heatsink (Plug in the heatsink's Fan to the motherboard)
5. Put 1 stick of RAM into the #1 Slot (usually nearest the CPU but check MB Manual for correct slot and also the number of Sticks RAM needed to power the computer) (make sure the tabs click into place holding the RAM in place)
6. Put your Video Card into the PCI-e slot (usually the one nearest the CPU but check the MB Manual to be sure)
7. Plug in any power connections needed for your Video Card (depending on the card it may need up to 2 power connections)
8. Put the Hard Drive back in the correct place inside your computer case, fasten with screws.
9. Plug in the SATA or IDE connection to your hard drive and then to the Motherboard (SATA 1 or IDE Primary)
10. Plug in a power connection to your Hard drive
11. Plug in the PSU's 2 connections to the motherboard (24 pin and 4 pin for newer computers) (1 x 20 pin for older computers)
12. Plug in Case Front Panel Connections to the Motherboard (Power SW, HD LED, Reset SW, Speaker etc...) (your external speakers will not usually beep if the Video Card or RAM isn't working properly so the Case Internal Speaker must be plugged in) (refer to your motherboard manual for connections)
13. Make sure your Case Fans are plugged in
14. Plug in Keyboard/Mouse, Plug in Power to the PSU and Wall, Plug in Monitor to the Video Card (VGA, DVI or HDMI) and Power Connection to the Monitor (then turn it on).
15. Once all of this is completed try starting the computer.
a. If it works Unplug Power then install 1 stick of RAM at a time starting the computer each time to make sure it still works.
b. If it doesn't work Unplug Power and Remove the RAM and replace with another stick. Plug in and restart to see if it worked.