Hi,
Thank you for writing to Microsoft Communities. From your problem description I understand that your power icon is disappeared from system tray.
1) Were there any recent changes made to the computer prior to the issue?
2) Do other icons appear in system tray?
I would suggest you to follow the steps provided and check if it helps.
Method 1: Try to run [System File Checker] SFC scan on the computer which would scan for corrupt system files on the computer and replace them.
http://support.microsoft.com/kb/929833
Method 2: Try creating another power plan on your computer and check if the issue persists.
Change, create, or delete a power plan (scheme):
http://windows.microsoft.com/en-US/windows-vista/Change-create-or-delete-a-power-plan-scheme
Reference: System icons do not appear in the notification area in Windows Vista or in Windows 7 until you restart the computer:
http://support.microsoft.com/kb/945011
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully.
For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry refer the link.
http://windows.microsoft.com/en-us/windows-vista/Back-up-the-registry
Please perform these recommended steps and post back if you are still experiencing the issue.