Hopefully someone will be of more help than I, but I did find a
temporary fix for you;

For many people with this problem, simply logging out and then back in
will restore the battery meter, at least for a while.

See also:

System icons may not appear in the notification area on a Windows Vista-based computer until you restart the computer:
http://support.microsoft.com/kb/945011

My battery meter stopped showing up on the toolbar.  When i go into the
taskbar properties and when i go into the notifications tab the power option
is unavailable.