Once Windows is activated legally then it should remain so - it doesn't have an expiry date where it no longer becomes genuine.
However, occasionally you can get the not genuine message.
To find the status of yours, go Start - type activate windows - press enter and see what message it returns.
To reactivate, open a command prompt as an administrator and enter these cmds -
net stop sppsvc
(wait until the service has stopped before entering the following lines)
CD %windir%\ServiceProfiles\NetworkService\AppData\Roaming\Microsoft\SoftwareProtectionPlatform
REN tokens.dat tokens.bar
net start sppsvc
slui.exe
After a couple of seconds the Windows Activation dialog will appear.
You may be asked to re-activate and/or re-enter your product key, or Activation may occur automatically.
If you are asked for your Key, use the one on the COA sticker on the machine's case
Then enter shutdown /r /t 00 to effect an immediate reboot and then redo "activate windows" to see what it reports.