Find your motherboard; click on the "Download" tab beneath the picture of your MB; select your operating system; then click on "BIOS " and download the most recent BIOS. If you don't have the user manual, you can also download it and follow the instructions for flashing the BIOS. Good luck.
Are you saying that you can't boot your computer because you tried to flash a new BIOS version and it failed to complete? If so, then you will need to acquire a new BIOS Chip from the manufacturer or buy one off of Ebay.
Please post your motherboard model number and maybe we can help you find what you need.