If you have the MAC address for your network adaptor that is like an electronic serial number that is hard wired into the network cards. The wireless and ethernet cards will both have one.
All you have to do to display it is to open CMD.exe (dos window) and type: ipconfig/all
I don't know where you would have it written on paper/manuals etc. You can almost certainly get a record of it from your ISP or manufacturer if you explain your situation and provide some sort of proof of purchase.