1-check that it is properly connected and the port is working.Try inserting in another port.
2-See if the files are hidden. To resolve this click on
tools---->folder options---->view--->show hidden files and folders.
3- may be the pd has no files at all. It only cntains previous used up sectors which occupy the space. Try defragmenting it.
4-finall, if nothing works, either your system or pd conatains virus. Try a latest antivirus solution.
hope that it helps:=)
Your problem suggests that the Memory Chip of the pen drive is corrupted or damaged. You can try to open the files using another system. If you are able to open them then backup all files and format the Pen Drive. It may solve your problem.
But, if the problem persists use any pen drive data recovery tool to recover data first and if it is in warranty take to service center for replacement.
I usually format my pen drive using linux for this. Try to format the pen drive (even using Windows). That should solve your problem.
Also, this can happen by a damaged memory chip of your pen drive. Try using branded ones like kingston, transcend and iball etc.