I was playing Minecraft some months ago on my Mac since my PC was broken. During that time I had to migrate to Microsoft, and it worked perfectly. Now I got my PC back and when I log in into minecraft.net it says that I haven't bought Minecraft. The launcher says I can only play the demo, but on my Mac I can still play the full version so I am very confused.
By migrating you have removed your purchase from Mojang's minecraft.net and moved it to Microsoft's XBox Store. In the launcher use the 'add account' function, choose Microsoft account, and use your Microsoft account login and password to link it up to the pre-existing purchase present on your M$ account.