Rule #1: There is no need to write anything to flash. You can run a boot image that is not in flash, so hold off writing anything.
Rule #2: If you've got the space, make a full backup on your desktop, then you're bomb-proof.
Do you have ADB working? Have you used UsbView.exe (presuming that you are running Windows? Linux is actually easier with lsusb.)
Step #1: Determine if you can get to EDL. This does no harm but you will have to do a LONG power press to restore usage.
Does it go off into voodoo-land but the screen remains the same (frozen)?
Does UsbView.exe show VID/PID 05c6/9008? That's EDL mode.
If you want you can use my UsbLog.exe which is found on my website up a level, down to the Windows section. It's quite handy to figure out where all the Windows "bongs" are coming from.