You don't say what computer, OS Version, or flash drive you are using but I have seen this happen upon occasion. The fact that it worked at first means that you probably have a good format on it although a friend of mine does format his flashdrives (8G or higher) with NTFS for use with XP and Vista (boo). I have had some USB connected drives that were stubborn and what has seemed to work for me is booting up in safe mode (holding down the shift key) with the thing plugged in the first time and then see if it doesn't come up that time or a second time when you boot up. I've also seen permission/preference problems mess up mounting external drives as well (I use AppleJack on my systems to fix such things).
Good luck with this and welcome back!
