Are you running XP? How do I clear past items from the Customize Notifications dialog box in Windows XP? Icons come up on logon, but not after reboot. Something to do with blocked processes with this model using XP since 2006. (Also a corrupt user profile has been known to cause the problem) System Restore will not be a permanent fix. You might want to check those out, maybe it'll help.
For what it's worth, the only time the battery level icon is shown on my Dell Laptop is when it's not connected to the electric power socket. Are you sure its not connected to the grid when the icon is missing??????????