Firstly you should not remove the MP3 player USB cable when your computer is using it - it may cause problems as you have found out.
The proper way is to remove an MP3 player is by using the "Safely Remove Hardware" control panel before unplugging your MP3 player.
Click on the Start orb (Start Menu), then click Run.
In the box that says "Start Search" copy and paste the following piece of text:
RunDll32.exe shell32.dll,Control_RunDLL hotplug.dll
Press Enter.
This will open the "Safely Remove Hardware" control panel.
You should notice your MP3 player listed in the list of devices. It might be called something like "Mass Storage Device".
Click on it and then click on the "Stop" button.
Windows will tell you that the hardware (your MP3 player) has now been stopped and it is safe to unplug it.
This web site explains the feature as it is in Windows XP but it isn't that different in Windows Vista:
http://www.helpwithwindows.com/WindowsXP/howto-07.html
Now to your problem. Vista still thinks that your MP3 player is still plugged in, although it isn't. Use the above process (safely remove hardware) to tell Vista that you have in fact unplugged the device.
After you have stopped the device in the safely remove hardware control panel, restart your computer.
When the computer has restarted, plug the MP3 player back in and it should be recognised again.
Remember to always use the "Safely Remove Hardware" panel next time you unplug your MP3 player.