Gamertag changed and new server profile (?)

Hello everyone. to briefly explain my situation. I reinstalled my OS on a new SSD, thus also reinstalled my Minecraft. Logged into the same account I had played on literally 2 days before and my Gamertag was different as well as when I connected to the server my mates and I had been playing on I was suddenly called "Leffeland" (?) and it seemed to have created a whole new server profile as all of my equipment had gone. Any help would be greatly appreciated.

Cheers.

:)