Hi there,
You may have accidentally told the program to minimize to the system tray instead of the taskbar. To fix this, please do the following steps in reverse: http://www.askvg.com/how-to-get-hide-window-when-minimized-option-back-in-windows-live-mail-in-windows-7/
Let me know if that helps at all!
Cheers,
Walker
Windows Outreach Team