a way to verify if there is anything wrong with the battery on the mainboard:
keep turning on your computer at least 12 hours, then turn it off; turn it on after 12 hours.
if there is nothing wrong, it means your battery on the mainboard is good. if it can't start, you must change a new battery. the battery on the mainboard is rechargeable, go ask sales when you buy it.
what 哈尔滨人 said is right. before you change CMOS battery, you better try another hd if you have one. but if you don't, it doesn't matter. just go to a computer store close to your home and change it, it is not expensive.