By power pack i assume you mean Power supply.
1) Are you sure you bought the correct power supply for your machine? For example Dell is notorious for using non-standard connectors on their wiring and you usually end up having to either buy adapters or a powersupply directly from dell.
2) Did you buy a big enough power supply for your computer? the more hard drives, ram, etc you have, the more power it draws.
3) Some motherboard/processor setups require more than one power connection to the motherboard. Have you verified that you have connected the power supply correctly?
4) Did you buy a quality power supply from a reputable dealer? Generally if you spent less than about $80 its probably not going to be very reliable. It may be defective out of the box.
Jim R
June 2008