Next step might be to change the cables... those ribbon cables and connectors tend to become brittle and even shrink after a long period of use... if the wrong wire pulls out or becomes loose, then nothing is going to work.. and they can become defective just from the dry air in the case.
New cables should run about $3.50 in most stores...but some will charge you $7.50 or $12... On eBay they can be as low as $1.50 plus shipping.
Remove and reseat cables, or exchange for new or from another known good working computer.
Exchange cables
It is safe to remove the CMOS batter if the computer is unplugged. Deplug the computer once more after a half hour or less, and replace the CMOS battery... or you can just buy a new one if you are in commuting distance of a Walmart or Best Buy or a tech store.... at Wal-Mart where I live, they are $3.50... Down the road a piece, another Walmart charges $4.50... Some will charge you $17... Our Radio Shack charges $15, needlessly. so you may want to shop around.
The batteries last for years... 7 years on average... but you just want the battery out so as to turn off the system BIOS.
When you put in the battery, you will have to reset the time and date and a few other things.