I recently wanted to reinstall Ubuntu on a partition on my PC. I used Gparted to delete the partition so that I could start fresh. I tried to boot to the usb, but it went into grub rescue mode. I looked at other solutions on here, and tried the whole ls->set root->reboot method, I tried it on each partition on the hard drive, to no avail. I then tried loading grub onto the flash drive and booting to that, no change. Then I reinstalled Gparted, and still nothing, I'm just stuck in grub rescue. I am at the end of my rope here, anyone have any solutions?
It sounds like your system boots to the harddrive but fails. When you say "boot to the usb", is that Ubuntu Live on a USB stick? If that doesn't work, have you tried burning a Ubuntu Live CD and booting from it?