While switching on PC CPU power LED not working,but power LED in mother board working,what the reason ?

By "CPU power LED" I suppose you mean the HDD LED for there is no such thing as a "CPU power LED". So, there isn't really any definite diagnosis for your problem because these symptoms are not connected. I am thinking, perhaps, you might have tinkered in side the computer and some how disconnected the HDD LED from the jumpers on the main board and then did something goofy to your graphics card (unless you have an integrated graphics card in that case you're probably screwed). You could've dropped it and caused these symptoms too though...anyways, here's what you can do. Carefully open the computer. Before touching ANY component inside the computer, tap the sides of the computer repeatedly. This will ensure that you are "ground" and will not pass on electricity that will surely damage your unit even further. After discharging yourself, go towards the back of the computer and you will see where the cards stick out the back. Most likely the card that is closest to the top will be your graphics card. Now carefully secure it and make sure it isn't loose. Also, make sure there is not dust build up on it. After doing that, focus your attention to the bottom right of your main board. Somewhere around there you should see multiple colored wires plugging into the board. Check all of them, making sure the connections are secure. That'll fix your HDD LED. That should just about cover it! Hope I helped!