I had the same problem, but after I had filled it up with data. I put it in the port and the drive would not come up. I checked Device Manager and It was in there, in fact everything told me that it was there and working properly, but it was not showing up in My Computer, so therefore I could not access it.
I did eventualy manage to reformat it and all was well. I can't remember how I get to the format option but I played around with it for some time and just ended up there.
Once formatted all was well. I have learned from this experience not to trust the Flash drive (pen drive)with any important data. Even today I am going to my sisters place to pass on some data to her and I am taking a backup with me.
If you can reformat then I suggest you do so.