yeah, it likes to go off for a tea break, often. MS may fix that, one day.
One simple answer is, reboot the system. You haven't 'hidden' it, have you?
There is, however, a more geekie way to get the Icon back.
Go to Start -> Run or press Win key+R, type in or copy and paste the following line and click OK. The command below
IS case sensitive.
rundll32.exe shell32.dll,Control_RunDLL hotplug.dll
That will bring up the “Safely Remove Hardware” dialog which you can use to stop the USB device that you want to unplug. When you close the window, this often has the side effect of also restoring the icon back to the tray.
saurce;
http://www.raymond.cc/blog/restore-missing-or-disappeared-safely-remove-hardware-icon/#ixzz2VMi7dNY8