computer came up first with cmos checksum error, then computer started slow to come on and came up into bios on the cpu processor to set manually. then it would not come on at all, but the stand by light stays on when you have the power on but the system will not boot up. I have checked memory, replaced processor, and checked the hard drive , all are working perfectly. Suggestions as to why it is not coming on at all. Help me please.

How old is your Computer?

Usually you get these errors when your CMOS battery is weak, or dead.

Some times but rarely it is also caused by Motherboard problems and failing power supply may also cause problems.

Check your CMOS battery, Power supply and check all connections to the motherboard. Check motherboard physically to make sure the board isn't cracked.

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RAM would have been my first guess, and I'm glad you know enough to check that first as well
Power surges can cause this, but you would likely notice a smell of burnt colour somewhere around the power supply.
If power is getting to your standby lights, then it's hopefully a good sign... most likely your motherboard is fine, i've seen them go on many models and i most cases you get nothing... absolutly nothing happens.
Although your computer is plugged in, that little battery can still run the show, it is the starter motor if you will of you computer, keeping track of more than jsut the time when you turn you computer off.

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