how to reset cmos

i am using compaq CQ40 laptop,when i switch on my machine power on & off. no display only fan is working. i went for my agent they told to me need to replace motherboard & cost will around 500US$. & i went different person who doing computer hardware. he told to me problem in cmos battery. & he was remove the cmos battery. now i using without cmos battery, next day i switch on the machine same problem came. let me know how to solve this problem, i think i have problem in motherboard,