It all kinda depends on whether or not the Bios chip is integrated on the mainboard, or if it is a removable chip. In the first case the only solution is getting the chip flashed again at an authorized HP tech center (and knowing HP, this can be quite expensive). If the chip is removable you have several options.
Try to get a replacement BIOS chip, or find someone with the exact same laptop and try to do the following, very dangerous trick (if something goes wrong, you end up with 2 broken laptops):
Swap BIOS chips (putting the working BIOS in your laptop). Start your laptop. Once it is started carefully remove the chip and replace it with the faulty one. Flash it.
Remember, this is a very dangerous operation and if you are not sure what you're doing, you could easily end up with 2 destroyed laptops.