This a very common problem with USB flash drive. Well, as per your description you had copied some files from another computer. This is a very common virus problem which hides all your data and makes pen drive empty drive when they are open from my computer. Generally, if are you are using Windows Defender or any other free version Antivirus, there are chances they might not found the virus. So these are the step to recover your data and get your pen drive working back.
1. Scan your flash drive using paid version or full version of any good Antivirus like Guardian. See if this help you out, if not then follow next step.
2. This is the most successful method in such type of virus attacks. Follow these steps carefully to get your pen drive back and this will work 100%.
2.1. Insert your pen drive into your system and see if it detect it.
2.2. If detected, open your pen drive and where it shows you "Empty Drive".
2.3. Open command prompt here by holding the Shift key and right click and selecting open command prompt here from context menu opened.
2.4. Or you can open a command prompt by pressing Windows key + x and then pressing A, if you are using Windows 8 or latest.
2.5 After opening command prompt make sure you are under your drive letter as root letter. Like if your drive letter is F: then command prompt must show as F:\>. If this is not as described above type your drive letter followed by colon, like E: and hit enter.
2.6. Type ATTRIB -s -h /s /d /l *.* and hit enter.
2.7. And open your pen drive on my computer. Whoaa !!! all files come back on the drive.
Let me know if this method works for you and do not hesitate to ask any help further.