yesterday I press the power button 6/7 times. then the pc start. it occurred many time. but now this time it not start. I check all connection and all parts. all things are ok. So what I need to do now?

Most probably the problem is in your power button. you can do this. Remove the wires from the button. then plug to the A\C current. After that just short that 2 wires for 1 second and keep it separately after shorting. If not start the computer just check that wire. if wire is OK, then check motherboard connections.

try to take the power cord out and hold power button for 30 seconds, or take out cmos battery and try to boot

Sounds like a button problem. Most likely, the connection is not good which is why it would take you multiple tries to power on the computer. One of the wires may not be making good contact any more. Open the computer and check the connections.

yup all the problem lies just with your power button only