That's one of those moments with tech of all types that comes with a Do Not Turn Off Until Finished warning.
I recovered the BIOS on a friend's tablet after some considerable research and a lot of trial-and-error and handed it back to her advising her that the BIOS and Windows would now do a bunch of updates to catch up.
The first time the update got going it mentioned wiping the boot record, she panicked, shut it down and it's now thoroughly bricked.
I could be wrong but my guess is your system has gone the same way. If you can't get the USB recognised, there may be little a dealer can do.