Dimension 9100, no video output

You don't say if you can boot the computer. Always the first thing to do on a old computer is to replace the cmos battery for less than $2. That retains the bios settings when the computer is disconnected from power. Check the manuals for some inspiration but don't expect too much and definitely don't spend much if anything on it. Test with used parts. The manuals will show if anything is missing--

https://www.dell.com/support/home/us/en/04/product-support/product/dimension-9100/manuals 

All Dell Dimension /Optiplex/Vostro etc have power button Light as well as 4 diagnostic LED's Dell XPS 400/ Dimension 9100 are the same machine.

Basic problem with No video is basic starting with Power Button indicator, then diagnostic Lights, then Beep codes etc.

Removing all cards and all ram and it should beep.

Off Blinking Amber Solid Amber Solid Green Blinking Green

The computer does not appear to get power from the Power Source.	The PSU is receiving power from the Source, but is unable to distribute it.	The PSU is working, but the system doesn't process data.	The system processes data and has begun POST tests. (See the diagnostic LEDs at this point)	The system is in a hibernate or sleep state.

https://www.dell.com/support/home/us/en/04/product-support/product/dimension-9100/manuals

 LEDS 1 2 3 4