My msi k8n neo platinum is not starting no power in mother so please tell me what should i do and whats the problem in motherboard it has no power when press start it and no beep no power tell me

Bad Power On switch, or bad power supply.
My money's on the bad power supply.

There is a way to bypass the Power On switch. Should you wish to know the procedure, state so in a comment.
(Believe upper right of page)

After you have made sure it isn't a bad Power On switch, check to make sure the 20-pin ATX motherboard power cable, is seated in the motherboard. TIGHT.

(Whitish see-through, long connector on motherboard.
Has 20 socket holes. 10 on each side.

Make sure the power cable is plugged in all the way, to this connector.
Make sure the lock on the side of the connector, is snapped in place.

(This one,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20

REMEMBER, computer unplugged from power! TOUCH the metal frame of the computer case, BEFORE you reach inside the computer.
Relieves your body of Static electricity)

Next, Power Supply.

When you press the Power On button, you are pressing a Momentary Contact switch.
The Power On switch that is behind the Power On button.
The switch is spring loaded inside, so that when you let go of the button, the switch opens up.
No contact.

When the switch makes contact for a brief second, a circuit is created. 5 Volts is sent through the motherboard, to the Power Supply.
The Power Supply turns on.

Power is sent to the motherboard. BIOS is the first program to be turned on.

BIOS 'see's what devices are installed, does a Ram Memory count, turns the Processor on, and hands the computer over to the Operating System.
(Windows XP is an example of an O/S)

Not enough power to turn the Processor on.

Your power supply has a weak voltage power rail, or a failed voltage power rail.

The power supply you have for your MSI K8N Neo Platinum, has three main voltage power rails.
1) The 3.3 Volt rail
2) The 5 Volt rail
3) The 12 Volt rail

Inside the Power Supply.
There are many components used inside a SMPS power supply, (Switched Mode Power Supply)
The main component that fails is an Electrolytic Capacitor, (Or more than one)

(Information about SMPS,

http://en.wikipedia.org/wiki/Switched-mode_power_supply

You can click on the photo to the top right of this page, to enlarge it.
Main thing I'm pointing out here, are the Electrolytic Capacitors.
The large one's on the left side, are used in the Input Stage.
The smaller one's on the right are in the Output Stage.
This IS NOT, an invite to open your Power Supply!! Read the info at the end of this solution)

Electrolytic Capacitors can fail. When they do, you have a failed voltage power rail, or more than one.

ALL the lights used in your computer use less than 1 Watt.
EACH fan uses 2 to 3 Watts.
A typical Processor uses 55 to 125 watts.
Depends on what Processor it is.

[The MSI K8N Neo Platinum motherboard has a Socket 754 processor socket. Uses AMD Athlon 64 processors.

An AMD Athlon 64 processor that fits a Socket 754, can use 51 to 89 Watts.
Again, it depends on which Athlon 64 processor it is)

Solution?
Use another Compatible, Known to be Good, power supply for a test.
When you have conclusive proof that it is a bad power supply, replace it.

NOTE*
As stated, Do Not open your power supply to try to fix the problem.
The Electrolytic Capacitors inside, can hold a charge for Weeks, sometimes Months!

Should your fingers touch the terminals on the bottom of a capacitor, OR, touch a circuit that one or more capacitors are in,
the Charge could be released to YOU!

Shock ranges from BAD to FATAL!

Best method is to use a substitute power supply for a test.
Be Safe.