|If you have EVER dropped the USB flash drive it may be DAMAGED.|
Your flash drive may not be detected properly when it's not plugged into a USB port that's built into the mboard, or when it's plugged into a USB port that is physically damaged such that the USB plug on it is loose in the port.
Troubleshooting USB device problems including for flash drives, external drives, external memory card readers.
See Response 1:
If your sound is on, you SHOULD hear the "dong-ding" sound when you plug it in, and the "ding-dong" sound when you unplug it.
If you DO NOT hear that for the flash drive, but you DO hear that when you
plug another USB device into the same USB port, the USB flash drive is probably DAMAGED and will NEVER work again.
It's COMMON for people to unplug a flash drive while Windows is running, which can DAMAGE data on the drive such that a drive letter for the drive does NOT show up in My Computer or Windows Explorer, or it does show up there but you can't access the contents properly, and/or there are files or folders not being shown that you know were being shown previously. .
See response 2 regarding how you use Safely Remove Hardware:
See response 2 regarding examples of which programs you could try to use to repair the data damage:
When the USB flash drive is plugged in....
Start - Run - type: diskmgmt.msc , press Enter
Scroll down the Disk Management screen
There should be a rectangular box with a title Removable at the left end of it, that has a total capacity stated at the left end that is a bit smaller than the capacity of the USB flash drive.
If you don't see that, the flash drive is NOT being detected at all.
If you DO see that, if it says RAW or unallocated in the rectangular box, data on the drive has been DAMAGED, probably because you unplugged it while Windows was running without using Safely Remove Hardware to STOP accessing the drive BEFORE you unplugged it.
Usually the data that that has been damaged is the partition table's data.
You cannot delete the partition on it in Windows itself due to Microsoft default settings.
You cannot successfuly just Format the drive, which Microsoft does allow you to do, or just Format it with any other program, when the partition table's data has been damaged.
See the link above for examples of which programs you could try to use to repair the data damage.