About a year ago, I bought a 16GB HP branded flash disk. (manufactured by PNY for HP) About 2 or 3 weeks later it developed the same problem you're having. And like you, nothing seemed to work...not even the HP utility, designed to fix this very problem...
Then, I stumbled upon the partition manager app I was already using: Easus Partition Master Free. It has a built in data / partition recovery tool. Oddly, the data recovery option has a limit on the amount of data it will rescue, (unless you pay for the pro version) but using the partition recovery option has no limitation and made my entire flash disk usable again! (And I'm still using the drive with no issues since then...)
The funny part was that I had previously used the same tool (about 3 years ago) to rescue an older 8GB flash disk that had been made unusable by removing it "unsafely" from a WinXP machine...and had forgotten all about it...in that case I still received some error messages, but the drive was readable and writable.
By the way, apparently this is an issue with flash disks larger than 16GB. As I stated earlier, HP is so aware of the problem that they made a special app to fix this problem. (which didn't work for me, or some of the other readers) My research at the time, showed that it had something to do with the way these newer, larger drives are made. I have to wonder about whether it also affects SSDs...