Re-assembling the PC

Now you are going to carry out the previous procedure in reverse.

Mount the CPU (take great care!).
Mount the RAM.
Mount the main board on the case’s chassis. Use the same precautions as you did when removing it.
Secure the board using screws. Make sure that the board aligns perfectly. Otherwise you could be shorting out the board and potentially damaging it.
Install the HDD, FDD, CD ROM, other storage devices, etc.
Reconnect all jumper-connected wires to the board (eg HDD LED, Reset, etc).
Reconnect the power cable(s) to the main board.
Reconnect the data ribbons (cables) to the HDD and FDD controllers. You might also need to reconnect serial and parallel data connections if using an older system.
Reconnect storage devices to their data and power connection.
Check that everything is connected correctly.
Check again.
Replace the case top but do not secure yet.
Re-connect cables and connections at the back of the PC.
Reconnect the power and turn on the computer.
Does it work? If it does, turn off the computer, disconnect the power, and secure the top with screws.
If it does not work, revisit all of your steps. Pay attention to the POST screen. This screen will give you valuable information.