If you only see Steve & Alex skins in the game, follow the instructions below to solve the problem:
1) Check the Minecraft skins server status. If the skins server is down, it is the cause of the issue that only Microsoft can resolve.
2) Open the mods menu by pressing the right-shift key and check if you haven't enabled the Nick Hider mod. If it is enabled, disable it or open the mod's settings and disable the "Hide Skins" setting.
3) Try re-logging on the Vanilla Minecraft launcher.
4) If the issue started happening after you had downloaded a new program(s), you might have downloaded incompatible software or enabled firewalls. Click here to see the list of incompatible software. After the uninstall, restart your PC for the changes to apply, then open the game.
5) If you have used third-party account generator tools, the issue is most likely caused by it. Switch back to the Mojang servers, then remove the tool from the system.
6) Run the Malware and Adware scan on your computer.