It sounds like your iPod may not be recognizing that it has disconnected from the computer or may have stopped responding in some way. I would give this article a look first:
Safely disconnect iPod
This section in particular:
If the iPod display still shows a screen indicating that it should not be disconnected as explained above, use the method below to eject iPod.
- Click the Finder icon in the Dock.
- Select the iPod icon.
- From the File menu, choose Eject iPod (Command-E).
- Double-click My Computer (Windows XP and Windows 2000) or, on the Start menu, click Computer (Windows Vista).
- Right-click the iPod icon.
- Choose Eject from the shortcut menu.
If the issue is still ongoing after trying that, then I would use the interactive iPod classic Troubleshooting Assistant: