Can't save any size file to an empty 8gb flash drive

Hello,

I've recently run into some issues with 2 of my USB flash drives. One is 8gb and one is 4gb and both used to work. However, recently i've been trying to move files to a Mac Desktop for school, but both flash drives won't work any more. They say there isn't enough room to store anything on either drive. I tried to get information from the drives but it says that there are zero bytes available. Is there anyway to get the drives back? It's very important that I can use them to transfer work (mostly video files) between computers for class.

Thank you,