Power but no startup!

As you know, it could be nearly anything.

Start by opening up the case, and checking that all plug-in cards and RAM modules are fully seated in their sockets.
Pay special attention to the AGP video card. It only needs to be a few 0.001" from being seated to cause a problem.

Next, look and smell for burnt or damaged parts on the power supply or motherboard.
Make sure that the CPU fan spins up properly.

Then, strip the system back to a minimum configuration.
Motherboard, RAM, CPU, Video card.
If the PC now boots, connect / replace devices 1 at a time until you identify the bad part.

If it doesn't boot, you then need to have parts to swap in.
Start with the power supply, video card, and ram.
Next try the CPU. If it is stilldead, it may be the motherboard itself.

1 last trick, follow the motherboard mfgs instructions for resetting the CMOS memory.

Chas

If your computer "sounds as if" it is starting up normally (you see all the leds and hear the hdd spinning), then probably the monitor cable has fallen out of the back of the computer. The screen is blank and gives a no-signal warning, which means it is getting power, but it does not see the computer.

If the monitor is securely connected to the computer, then open the computer (powered off of course) and make sure the video card (if it is separate from the systemboard, has not come loose.
Generally if a computer has been working fine, and still does not give you extra beeps on startup, it's probably nothing more than a loose/disconnected cable.

Chas has pretty much covered this situation.
AGP video cards do have a tendency to rise up in the slot. Another thing is hopefully your CPU fan hasn't stopped working. AMD chips run very hot and will burn up allmost instantly without cooling.

You say all lights are on. Is you pc actually running? can you hear it? If so, and it is only the screen you can't see, then it's very possible your video card came unseated.(it doesn't take much). Pull it out and then reseat it, check your cable attachment and cable to the monitor itself, make sure they are O.K., Then re-boot. If you can, try swapping a different monitor and see if that one works.
Good Luck!
Jeff

Hi I have seen this many times on a PC when HDD led stays on the PC is not even getting through or to the POST and cannot issue any beep code usually this is a small compatability prob in the hard ware. Reset the CMOS with jumper or removal of battery for 10 Minutes, then power on do not set cpu settings in bios if it is soft cpu settings go to mobo web site and do latest bios upgrade. This should cure futre problems.

Rich