Hi and welcome to the Gigabyte Forum.
It could be more or less anything that is causing the problem at this point. We shall have to try and narrow it down to start with. I think the best thing is to remove the motherboard from the case along with the PSU, CPU/hetsink/fan, one memory module in the first slot and buzzer. Sit it all on top of the motherboard box if posssible and be careful of static electricity.
When ready try booting and see what beep codes it makes. Post back with them.