For those who lost their windows 10 digital entitlement product keys:
There isn't a need for product key as it is a digital entitlement, the product key saves in your motherboard, no need to worry.. So the next step is to reinstall Windows 10 by media creation tool. You should create a USB and boot it, In the middle of the windows 10 installation, right in the product key step, press the skip product key button and after installing and connecting your Microsoft account (or not) and check your system information. It should say that it is activated. In the Anniversary Update, the product key will be licensed and saved in your Microsoft account in case of any hardware failure and you should reinstall it, no problem, just connect your Microsoft account, it will be activated.
Hope it helps!!..
Posted via the Windows Central App for Android