Right click on my computer click on manage , select disk management and see whether your pen drive is showing 0 bytes unallocated. If it is showing like that right click on the drive and create partition with maximum size.
I was having the same problem. After much search and try, I found the solution on following link
You just need to add your email and country and it will recover the data if possible. If recovery of data is not possible then atleast it will format your usb and make them able to work again
You may change a computer to insert your pen drive to see whether the same situation will happen. And confirm your pen drive is not encrypted by some software. Otherwise your pen drive may suffer crash or corruption. You can use some repair tools to fix it.