This might be worth a crack.
Open Terminal* and find its name with;
Having noted its name, type.**
chmod 775 /Volumes/itsname
*In Terminal, a command string is typed and then one presses the return key.
**note: there are only two spaces in that second string, either side on the number. Substitue the actual name after /Volumes/
That should return it to a read write permission and if so you should be able to reformat it with Disk Utilitiy's 'Erase' if necessary.
The name in terminal is "USB". In Disk Utility it is called disk5s1. When I execute the command "chmod 775 /volumes/USB" nothing happens and the erase button in Disk Utility is still unclickable. So is the "Repair Disk" button in First Aid and the "Restore" button in the Restore tab of Disk Utility. When I verified the volume, it said:
Verifying volume “disk5s1”
Invalid BS_jmpBoot in boot block: 000000
Error: This disk needs to be repaired. Click Repair Disk.
The repair button still wasn't clickable.
That error message "Invalid BS_jmpBoot in boot block" means the USB's firmware is corrupted, so it can't even mount to perform any operations on it.
I think it is beyond recovery. You could try another Disk utility application like Drive Genius or Disk Warrior, but only as an academic exercise if you already own one of them, because the USB flash drive is a relatively low value item. That is a silver lining to your USB loss I suppose.