Did you open the wrong configuration? When you launch Hammer you probably see a menu asking what game you want to map for:
This will determine what materials, models, entities, etc. get loaded. If all your content is missing then it's likely that you accidentally clicked on the wrong game.
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble