grub loading stage 1.5 error 15 error message does not necessarily indicates that you have destroyed a MBR ( master Boot Record ) of your booting partition. In fact this is a very common Grub error message. Usually this grub problem is resolved by changing a BIOS boot hard drive priority.
It may happen that you just left your USB stick inside your PC and BIOS boots from it because it is set to boot from USB devices as default.
Another possible cause for grub loading stage 1.5 error 15 error is that you have added or removed some hard drives from your computer which changed the BIOS's boot order.
Grub stage 1.5 is optional stage and is 30 KB long and starts right after of MBR. This stage describes file system used on the partition and loads grub's stage 2.