It's possible to for example visit a malicious website that uses a vulnerability in the browser to jump out of the browsing sandbox to the OS then use another vulnerability in the OS to get kernel level access and then execute the instruction to overwrite part of the BIOS making the system unbootable.
The only fix then would be to either get a new BIOS chip or own a motherboard that allows you to flash the BIOS using zeroboot via a USB stick. Some Asus motherboards support this feature, my own one does and many of their newer boards from 2012+. Basically you load the BIOS firmware on a USB flash drive and at the back of the motherboard is a USB port clearly labelled for BIOS updates only, it's usually flipped vertically from the other USB. You put the stick there, apply power to the board (but it doesn't need to be booted or even have RAM or a CPU installed) and then hit the Flash button at the back of the I/O panel.
I'm glad Asus added that functionality. Also if you have a motherboard with IPMI (this is rarer on consumer stuff, mainly found on server and workstation boards) you can flash the BIOS over a network connection through a webpage hosted by a daughter board / auxiliary chipset on the motherboard and that too would fix the BIOS after being corrupted.
Another safe guard if you have a board with dual BIOS chips, many do now, including my own, most of Gigabytes and Asus's boards include this feature you can simply hit a button on the motherboard or switch a jumper to switch chips allowing you to re-flash the broken chip. Just obviously don't boot back in to your operating system until you've reflashed the bad chip incase whatever caused the problem is still there.
And finally you can just buy a new BIOS chip pre-flashed with your motherboards firmware. I've done this in the past for a server board that was too outdated for me to update as it didn't support the CPU I intended to use in it, this is the company I used and they charge about $13 for a chip which is small potatoes really: http://www.bios-chip24.com/
they had the new chip to me within 3 days and it worked perfectly, had the exact firmware on it that I requested and booted fine.