I assembled a new PC ,when I push the on button every thing works but the desktop won't turn on once when I turn off and switch on it does work perfectly! This happened around several times please help me??

If Im understanding correctly then it may be that you have the wrong type of switch installed on your computer. If it is an on/off switch then you shut down your machine with the siftware it will take 2 pushs of the button to restart it. Is that what is happening ?

Double check your connections from the front of the case to the motherboard. Keep in mind that lights (hdd activity led, power led) are positive negative specific and should be plugged in correct polarity. On the other hand switches like power or reset are just breaks in the line and are not polarity specific. Make sure that the case diagrams for the front wiring are properly placed on the correct pins according to the motherboards manual diagram. Sometimes you have to use a magnifing glass to see the + and - symbols on the board if they're even there. So correct polarity and correct pins. I've had motherboard manuals that seemed to show the pins upside down but typically there will be one corner marked as pin 1, that helps in cases of confusion