I had this problem when updating drivers a few times on my HTPC rig. Do you at least get the HP logo and/or initial startup information showing on the screen when restarting, but when it goes into Windows it just blanks out? If so, as the system is starting, mash the F8 button repeatedly until you get the boot menu with options to go into Safe Mode. You may need to use that to reset your video settings (usually a bad resolution gets locked in or some other holdover from a previous version of the video gets stuck).
If you don't even get the BIOS boot up information, or any action at all on the screen - well, as Mark56 indicated, you may have a flubbed BIOS install.
One other question: Are you using an integrated video connection (part of the motherboard), or do you have a video card installed? Sometimes the video will switch to integrated after a BIOS update, and need to be switched to the PCI-e video output. Just something else to check on.