Many many many many many many things to consider here, so this is a bad question to ask here, but a few things that stand out are:
- Ensure all cables are connected properly.
- Make sure you did not zap the hardware with static electricity, if you did then you will have to replace it.
If possible disconnect any hardware that you do not need for the first bootup. If you have several Hard Drives disconnect all but the primary one. If you have several disk drives, disconnect all or just all but one.