Hi Jojo,
I had exactly this trouble this week. Are you using a university computer? The IT department told me that this happens if you have (even if only occasionally) removed the USB stick without closing it down first (guilty, in my case) with the green arrow at the bottom of your screen.
The USB hardware recognition on your computer will need to be uninstalled and then re-installed. If you are the computer's administrator then you can do it; otherwise, contact your IT department.