My Optiplex 260 was upgraded with memory and an additional HDD. The upgrade caused a CMOS change; that change apparently interefered with systems settings established when an AGP Graphics card was added to the machine 3 years ago.
Result: This Dell Optiplex 260 computer would not POST or boot. The video was blank black. Drives did not start.
The motherboard had power, fans worked, etc. Resetting CMOS had no effect. I was thiinking the machine was dead at processor or mobo level.
When the AGP Graphics add-on board was removed, and the monitor attached to the original on-board graphics connector, then the boot process detected the monitor, completed POST and completed the boot process.
Now I have to figure out how to reinstall the AGP Graphics board.
Moral: When trouble shooting machines which appear dead after an upgrade - possibly due to CMOS changes induced by the upgrade - consider the possibility that the machine has lost the ability to communicate with the monitor through an add on graphics card, which has been "lost" to the CMOS, and that this is why boot does not occur and monitor is blank black.