What kind/size power supply do you have?
If your power supply has a power switch, make sure it is turned on. If you PS has a voltage adjustment make sure it is set correctly. (in the US 115 is standard)
If all that is fine....
The faster more beefy processor like the AMD Athlon 900 will require more juice to function happily than your old K6-500.
A couple of troubleshooting things you can try:
Disconnect everything from the motherboard, RAM, IDE, any PCI or AGP cards, processor, etc. So that all you have left connected is the power supply connected to the motherboard, and the power switch connected to the motherboard. Then see if you can fire it up and get power. All yo uneed to knwo is if you can get power to the board so what you are doing is reducing the power load to see if the power supply can power just the motherboard.
If this works, then start adding components one at a time starting with the processor, firing it up in between each addition. If it stops firing up after a particular component or after you have everything put back together, I'd say invest in a new power supply, at least 300Watts, i prefer Antec or Allied brands, they both hve good warranties, and see if that resolves your problem.
