virtualbox.org

Until today, I have always seen the popup menu at the bottom when in seamless mode, where I can change the mode and mount devices. Now it's missing. I haven't changed any settings. How can I get this back? Any help is greatly appreciated - I need to mount my CD drive!

If you did not turn it off then you may have to reboot the guest and or the host to get it back. Until you find it though you can use the (host + home) key combo to get the menu to do what you need to do.
Check the VBox program GUI under settings ->General ->advanced and make sure that it still has the check mark in mini toolbar.

In a different topic I confirmed that the mini-toolbar doesn't show in seamless when it's at the bottom. Change the setting to it's at the top. You will notice something weird there, but at least you have a working toolbar.