Hi Crispian,
No problem at all, I can guide you through this next step. The activation key is not required for the software to install itself on your machine, so there's a good chance the installation was completed.
First, you can search for the program directly. Press the [Windows Key] + [S] to bring up the search bar, and then type in "Microsoft Flight Simulator". If the program is installed, it should appear in the search results. You can right-click on the program and choose "Pin to Start" or "Pin to taskbar" for easy access in the future.
If you don't see the program in the search results, there are a couple of additional steps you can take.
Step 1: First, check your Program Files. Navigate to your PC's Program Files folder (usually C:\Program Files or C:\Program Files (x86)). Look for a folder named 'Microsoft' or 'Microsoft Flight Simulator' and see if the program exists there.
Step 2: If you can't find it in Program Files, try looking in the Windows Apps folder. Please note that this is a hidden folder, so you will need to show hidden folders first. The Windows Apps folder is usually located in C:\Program Files\WindowsApps. If you find the program here, you can create a shortcut to your desktop or start menu.
Step 3: If you can't find the MSFS 2020 files in either location, you might need to re-install the software. I know this isn't ideal, but sometimes the installation process can encounter issues that prevent it from completing correctly.
Remember, there's no need to worry about re-entering the DVDs. The MSFS 2020 installer should be able to detect the files already installed on your machine and skip to the point where you were interrupted.
Once you've found or reinstalled the program, you can follow the steps from my previous post to activate your copy of MSFS 2020.
If you have any other questions or need further assistance, I'm here to help. Don't hesitate to ask. Enjoy your flights!