Just confirming, Albin...
- You have an XD-V75 receiver that worked fine when you "re-flashed" your (older) XD-V70 receiver, which is currently OK.
- That (newer) V75 receiver still seems OK and ready to update other equipment via USB and "Line 6 Monkey", ...BUT...
- Your THH12 handheld will not "light up" and switch on since a "re-flash" that went wrong.
If all of that is correct, here's my "take" that only expands a little on Don's reply.
First off, I would suggest that there's nothing to be lost by starting again with a new pair of absolutely fresh AA alkaline batteries correctly loaded into your "dead" THH12 handheld.
I would think that, even though your handheld seems "dead", you can probably effect another "re-flash" anyway, ...by placing it (again) over the XD-V75 "update interface" receiver's antenna exactly as illustrated in this document http://line6.com/support/docs/DOC-2583.
Having first re-booted your computer as Don suggests, you must then (once again) very carefully follow each step of the USB connection and V75 receiver "power on" procedures set out here, http://line6.com/support/docs/DOC-2585and (with any luck) once your XD-V75 "update interface" receiver is in "Loader" mode, it will then establish communication with your handheld upon you "choosing" it as the "target" device following the prompts from "Line 6 Monkey".
I'm hoping that you will then be on your way to a successful "re-flash" that will give your THH12 access to extra features and both RF1 and RF2 frequency schemes, almost "changing it into" a V75 handheld.