this may be an issue with the USB ports. I have had Pavilion systems not boot if things like mice were plugged into the front ports. This can happen after a power spike injurs the port. Unplug all external devices, including your USB mouse.
If not, then your video card may be gone. If you have a video card to test with, and know how to install it, I would try this route. At least you should be able to see enough to get into the BIOS.
Do you know if anything was 'odd' last time it worked? If so, can you explain it (to the best of your memory).
Aside from testing the video card, Hal brings up some good points. If you cannot install another video card, try Hal's suggestions.
My Pavilion media center has these, an on-board card (with a plastic cover because it was ordered with an upgraded card). A tuner card (useless right now), and an add-in card. If you had your card upgraded, it may be similar to my setup. Try removing the plastic cover (if you have it) and see if the video works from there.
Also, on my nvidia card, it has both s-video and composite video ports. try outputting using one of these instead (to a tv).
One of these options should work, if not it may be your motherboard is going out (USB issue) or power supply is damaged and/or not outputting enough power.
If none of the suggestions worked, try unplugging a few devices internally (if you have the knowledge to).
BUT, If it is under warranty, call for support before opening the case for any reason.