I was installing Lubuntu on an external flash drive (not wise) and then when I go to boot back up on Ubuntu this pops up.
Like @guiverc suggests, it looks like you accidentially installed the thumbdrive's bootloader onto your harddisk.
Does your PC boot Lubuntu when you reconnect flashdrive#1 (the one you installed Lubuntu to) but set your BIOS to boot from your ubuntu-harddisk?
If yes, remove your Lubuntu-Drive again and reboot to the error message. From there you can use this guide:
to restore Grub into booting Ubuntu. It explains how to use the grub cli to find your original Ubuntu-installation, boot into it manually and reinstall grub.