Edit: Welp, I re-made the live USB and now it works fine. Thanks for the replies! Another computing lesson learned the hard way.
I was messing around with my new dual-boot (win7 and the latest ubuntu, 12.04 I think?), and I deleted some partitions that I shouldn't have.
When I boot up the machine, I get:
error: no such partition. grub rescue> _
I know that I could probably fix this by booting from the live USB. But I can't boot from the live USB either! On startup, I can get "boot from drive" options by pressing F12. (This is how I installed Ubuntu in the first place.) But when I select the USB drive, I get this error:
error: no such device: [long string of letters and numbers]. grub rescue> _
I've searched around for a fix but most threads addressing the first problem suggest booting from live drive. And I can't do that!
Any help much appreciated.