Hi, I've just read your post. I guess the mobo tries to boot on usb device, so go to the bios and disable "usb device boot first" or something like in boot tab.
And check that"usb legacy" is enabled into usb in onboard devices tab.
Good luck !
I discovered the problem. It was an intermittent SATA cable fault - the cable connected to the boot dtive. When the cable lost contact the Bios could not locate the Ntldr file. A new cable solved the problem.