Hi there,
You need to RMA the motherboard through the place of purchase, if you are in the UK.
If I remember correctly the motherboard has a 3 year warranty from date of manufacture and the seller is obliged to process any claim, during this period. If, however, the motherboard is not found to have a fault, you may have to pay a handling/postage charge.
I work in retail (not for Gigabyte) and am amazed at how many sellers try to avoid their obligations, when it comes to warranty, despite the laws that are applicable.