when I press my power button 3 or 4 time (on off) or some time more then display show in monitor. when I press 1st time then the pc start but no display show in monitor. when I off it and then again on it ( 3 or 4 time I did it) then display show. but pc not start normally. it show a display like:
press F1 to run setup
prss F2 to start default operating system
and show with other more thing.
when I press F2 then windows start normally. now what I need to do?

Is it a PC or a laptop?

desktop pc

try turning monitor on after you turn on the tower

When the system indicates to press F2 to continue then it must be the problem with the battery in the motherboard you can change the battery which is called CR2032, then the probelm must be solved for you.

I change the bettery. but the problem not solve. one thing I unpluged all thing (motherboard, hard drive,all all) from casing and put it another place. then I start it manually ( i connect the two pin of start...) then it start 1st time. no need to connect it 1 more what I said above. plz help me.

If it works correctly outside the case, it may possibly be the on/off button/switch that is in the case.
You may have to try putting everything back in to the case and try it again.
Can you get a new switch for the case?

no. another thing what about "press f2".

f2 is bypassing any bios/setup errors ,hit f1 and run setup/bios ,make sure time and date are correct ,set bios to defaults,save changes and reboot

in bios the date and time are not ok. i change it many time but some it show correctly some time not. from yesterday when I connect the power pin manually it occurred same porblem again. (when I connect it two or three time then start)