The issue occurs quite often when I have my iPod connected to my laptop and I put the laptop to sleep. Even though the iPod is still connected when the machine awakens, it refuses to mount. The iPod continues to flash the "Do not remove" message and there is no way to unmount it. I'm fearful of removing it from the laptop, given the flashing warning not to.
A simple work around that I've found is to quit iTunes (if it's not already) and relaunch it. Upon launch, iTunes automatically looks for the iPod and successfully mounts the once missing iPod. Once this is done, you can safely eject your iPod as normal.

