|The flash drive (or external drive, or memory card reader) must be plugged into a port it will actually work in.|
See response 1
How to tell if the flash drive is properly recognized as a hardware device, even if you can't access it's data.
Programs you can try to recover your data, if that's okay.
See Response 2
The most frequent reason people can no longer access the data on a USB flash drive, USB External drive, or a memory card, is they unplugged it while Windows was running WITHOUT clicking on the Safely Remove Hardware icon in their taskbar lower right, and choosing to STOP accessing the drive.
(If you get a message you can't do that, then stop accessing it elsewhere - e.g. in My Computer or Windows Explorer, or in whatever you were accessing it with, e.g. change the drive letter, then you will be able to STOP it in Safely Remove Hardware.)
The Safely Remove Hardware icon may be hidden - if so, you have to click on < at the left end of the icons in the taskbar to reveal it.
It's a gray rectangle with a green arrow on it in XP, 2000, and ME, (and in 98 and 98SE, only if you have third party flash drive etc. drivers installed, or you have connected an external drive.)
In Vista, and probably Windows 7, it's a medium green small circle with a white checkmark on it.
If there is no Safely Remove Hardware icon, then there's something not right with the software that is supposed to make the icon appear (e.g.that sometimes happens in 98 and 98SE) - you could try rebooting, but if the icon still does not appear, then DO NOT unplug the device while Windows / the computer is running.
You can unplug the device at any time when Windows / the computer is NOT running. You can plug it in at any time.