Hi,
How did you know that the Windows Search Search Service is turned off? Is it that you got an notification/error message that told you about this when searching in Outlook?
To enable Windows Search Service, press Win key + R to open the Run command, type
services.msc and press Enter. Find the Windows Search service and make sure the Status is Running. If not, try starting the service. If it's already running, try stopping it and then start it again to see the result.

Hope this helps.
Regards,
Steve Fan
Please remember to mark the replies as answers if they helped.
If you have feedback for TechNet Subscriber Support, contact
tnsf@microsoft.com.