The Green lite on the board was it out when you installed the ram ,If not you may have fried something ,did you remove the power cord and drain power from computer before you installing the ram!!!

yes i did. the led thats not on right now is the 'stb led' the strange thing is when i turn the computer on i can still here the power supply hum and also i can see the lights on cd as its booted. but nothing else happens.

Sometimes when installing ram you can loosen or nockoff wires check and make sure you didn't pull off any wires ,or hope you didn't short the ram or board with a charge of static electricity

i think there may be a power supply problem maybe thats why the 'stb led' is not being lit up
is there any way to reset the power supply?

Not that Im aware of ,HP tech support woluld say that unplugging PS and Holding the Power button in for about 40 seconds drains and resets it i think !

Caperjack's suggestion is one thing to try, and here's another:

Sometimes power supplies have circuit breakers in them. Try simply unplugging the system for 2 minutes.

If that doesn't work, disconnect all external devices, unplug all of your drives, remove your RAM, and remove all PCI/AGP cards. Then, see if you get a light. If not, then you've more than likely got either a problem with your power supply or the motherboard. I hope you're still in warranty!

On Dells, that light is called a "flea power" light. It generally indicates that the ATX connections on the board are supplying power, so when you hit the button, it should respond.