Upgraded from Windows 7 to Windows 8.1 Pro, lost office programs

I just upgraded from Windows 7 to Windows 8.1 Pro, and noticed that I lost Microsoft Office (word, excel, powerpoint and one note). I bought a single use office a month or two ago and had it installed on Windows 7, and threw out the disk (including the product key) after installing it assuming I wouldn't need it again. Is there any way I can reinstall the programs without having to buy another copy?
Thanks