Hattori solution is the 1st way. If you cant understand command prompt then here is the easy way.
1) Right Click->explore then copy out all folders & files
2) Right Click pendrive again-> format
3) The pendrive is work now. Then scan all the folders & files that u move out just now and paste back to ur pendrive.
Done!
cannot open pen drive/no autorun, need to right click n open