Had a hard drive failure. Office wants a key to start up after backup. Was preloaded when I bought the laptop. How do I get activation key?

I had a hard drive failure on my MacBook Pro last week and had it replaced by Apple. When I restored my computer with a Time Machine backup, Office 2011 wants an activation key to start up again. Problem is, I had Office pre-installed when I bought my MacBook by Apple. Where do I get the activation key to get it working again?

Thanks

Tiffany Thomas