Sounds as if it got corrupted when you removed it.
A friend of mine had to do that at a library once, because the stupid librarian wouldn't help him remove it the usual way, via eject flash drive. There's a chance of corruption if it's removed without doing it that way.
Slightly OT -- I'll never understand why my local library doesn't have the normal eject flash drive option available... it would save a lot of people a lot of hassle and hair pulling.
RIP Robin, you'll be missed
. Credit goes to Ticketmaster for this picture of Robin, no copyright infringement intended.