This could be a number of things, such as:
1. You have not connected it back up properly.
2. One of your new components is broken.
3. One of the components is not connected properly.
4. Your PSU is not powerful enough for your new setup.
5. Your motherboard is not seated properly in the case and is shorting.
The simplest thing you could do is make sure all the wires are going to correct place (you can find a wiring diagram for your motherboard online) and that all components are properly connected. If that does not work check that you have put all the motherboard screws onto the case and that it is attached properly.