I think you have that backward, for anything more than 4GB, you want memory remapping enabled. It should explain this in your motherboard manual.
So are you saying that it was working and when you disabled memory remapping, it won't boot. That's weird, that shouldn't cause it not to POST.
Have you tried resetting your BIOS? The process should be detailed in your motherboard manual. Depending on your motherboard this usually involves unplugging your computer, moving the CMOS jumper to reset it. Just have a look in your manual.