This is most likely to be a malware and if your data is really important it's best to contact a specialist, so that you minimise the risk of data loss. On the other hand - if you don't need the contents you can perform a full format on the drive, but first of all make sure that your PC is free of viruses or malware. There are many third party softwares that can help, but I can not give a particular advice on that. The best solution of your problem that I found in a reliable source is this:
Click on "Start" -->Run --> type cmd then click on OK.
If your flash drive has a letter G:
Enter this command.
attrib -h -r -s /s /d g:\*.*
You can use the above command --> Right-click in the Command Prompt and paste it.
Note : Don't forget to replace g: with your flash drive letter if it is any other.
Now press "Enter".
Now check for your files.
After that, it is advisable that you download a trusted malware protection software and perform a full scan on both your drive and PC.
Hope this was helpful, best regards, Citizen_WD