When your USB is not recognized or accessed by PC, the boot sector might have been damaged.
1, if you have important files inside, you should find a software to get your files out of the not recognized drive.
You may follow guide which helped me when my SanDisk Blade 4GB USB Flash Drive is not recognized by my PC and report not formatted error.
Here it is:
2, if you do not have important files inside, just format it. You may right click the drive for formatting, or use disk managment to format it: right click my computer icon >> go to managment >> disk management >> and you will see your usb as external >> right click to locate the format option