Hi NielsD, when updating the BIOS we usually recommend incremental updates; you may skip few versions (two or three perhaps, depending on version number) but it is better to continue doing the update in increments in order to avoid this type of issue. The Intel® 6 Series motherboards have specific update instructions that were also discussed here: https://communities.intel.com/thread/30813
Considering the description you provided I believe the only thing left to do is checking if the motherboard is still in warranty so you can contact Intel® support for replacement.
I am truly sorry for this experience and inconvenience.