If I understand correctly, you want to play MSFS 2020 without any internet connection. Unfortunately, this game is made to work only when it has internet connection. For that there are some reasons: updating scenery while flying, AI traffic, real life speech, weather, but most important the security check.
So, the first time you installed the game, everything worked while being offline. Then you switch the computer off and after 1 month, you switch the computer on. Then MSFS 2020 tries to detect internet connection and cannot find it.
If I forget to connect with the internet, I get a warning, that I need to insert the game disk. This is for me a sign, that I forgot to connect with the internet.
If you have set everything for playing this game offline, the game still wants to check its validation before starting. This validation changes over time and your version has an old validation, the game “knows” and CTD. because it thinks it is pirated.
So, when you reinstall the game, at that time, the validation is correct and actual, so the game will start.
And this is - in my humble opinion - what is happing in your case.
There have been other gamers, who tried to play MSFS 2020 offline. But, they could not. There are 2 threads in this forum about this:
"Offline play not available? Offline play not available? - General Discussion & Community Support / Tech Talk - Microsoft Flight Simulator Forums
and “How to use the app offline?” How to use the app offline? - General Discussion & Community Support / Tech Talk - Microsoft Flight Simulator Forums