If you accidentally attach a device to sys-usb that you shouldn't have, Qubes may refuse to boot.
Here's how to fix the problem:
1) Boot into Recovery/Rescue mode from the Qubes Install USB
2) Qubes will mount your drive after you enter your LUKS password
3) Edit your /var/lib/qubes.xml file to remove all entries from "pcidevs=" for your sys-usb (note that it is not sufficient to turn off auto-starting, the system still will not start)
If Qubes Recovery/Rescue doesn't automatically mount your LUKS drive
1) Boot Fedora on live USB
2) Follow the instructions here to mount your LUKS-encrypted disk https://alvinabad.wordpress.com/2012/09/22/how-to-recover-a-luks-encrypted-disk/
3) Edit your /var/lib/qubes.xml file to remove all entries from "pcidevs=" for your sys-usb