Can not start after upgrading my hardware

I just upgraded my hardware after that MSFS wouldn’t start at all.

Ok we’ll need some more information than that. What hardware, and I assume it has started before? What changes exactly have you made to the system?

Changed the motherboard and CPU and added RAM

So it’s basically a new PC in which you inserted the hard drive of another PC.

What does “wouldn’t start at all” mean? Nothing happens after doubleclicking?

Ok those are pretty big changes. So first make sure you have a bios update for the motherboard, and make sure everything is plugged in correctly. Does the game just not start at all? Like, when you click the icon to start it, does it not bring anything up? And how much RAM did you put in?

Is windows Updated ?

yes it’s basically a new PC.
nothing at all happens when i double click

Make sure windows and all of your drivers are updated, including the GPU. Then, if it still won’t work, you might just have to reinstall it.

everything is updated and i have tried other simulators like X-plane and they are working fine

I’d visit your profile in the Microsoft store and reassign the computer.

irrelevant

Because it’s essentially a new computer, you have to treat it as such. Meaning, you might have to reinstall stuff or just start over from scratch. Simply attaching an old hard drive doesn’t mean everything previous will automatically work great. MSFS is very advanced and has a lot of technical things going on with it. I wouldn’t expect it to behave the same as other programs.

I bet MS is considering it a new PC with those changes. First thing you should do is check your activation for windows. Since the sim is tied to the gamepass app it will most likely not allow you to run games until it is re-registered with MS.
When I did the same a while back I actually had to purchase a new windows license due to a eula change from MS. Evidently now certain hw changes are considered a “new” PC and they require a new license. From what I take if it is minor like changing or adding an hdd ssd or graphics card they will not require it but changing you MB is major and unfortunately I think MS will require a new windows license. They nixed the transfer option after the free upgrade window passed.
Good news is you can transfer you live acct over no problem but you may be stuck until you can get a new windows license if you took advantage of the free Windows 10 upgrade a while back or you have the upgrade version. If your old comp had a full retail version you can transfer the license.

If above is not the issue you probably need to delete the config file for flight simulator as it is trying to load on HW no longer there.

Microsoft Flight Simulator is currently not available in your account. Here’s the error code, in case you need it: 0x803F8001
i got this message when tried to start it

If you look up that error code in google, (which is what I just did for you), it seems to basically be an issue with the store not recognizing you, similar to what @AZRedneck33 said. You will need to try and fix your account in some manner.

Are you logged into your MS account in the MS store?
You could try making your User as an Online account if you set as Offline at install

Is the Xbox app installed on your machine? Are you logged into it if so?

Yes its about windows activation.
Thank you all.

Unfortunately it is not as simple as just removing the device in MS acct. Honestly at this point it might be easier to just buy a new license since deactivation i believe has to be done on the old config. Unfortunately the old days of using the same Windows copy for multiple upgrades appears past. MS finally caught on and started restrictions on transferring and made the process a nightmare.