I restored my mac to factory settings, now mac states there is no OS X installed?

I restored my mac to factory settings to erase all data.

On restarting the mac, it prompts me to sign in with my Apple ID to install the operating system OS X Mountain lion.

When I sign in, it tells me that I have not purchased OS X Mountain lion.

When I purchased the mac the operating system was pre-installed, therefore I have never purchased the operating system through the App Store.

Therefore, I have no option to continue to turn the mac on.

How do I get around this problem as the operating system does not show up as a purchase?

Thank you in advance

Hannah