Good to hear it's working again! As the solution seems to be spread across several posts, here's a recap of the info:
Joining and leaving the Family Mix can only be done through an iOS or Android device, but it does apply across all devices afterwards. If you accidentally left the Family Mix, the playlist will still be visible but it won't contain any tracks.
1. Use your iOS or Android device to navigate to the Family Mix playlist
2. Tap the "Join" button that should be visible
3. If you don't see the button to join, tap the button with three dots to see options, then tap Join Family Mix
If it happens to be that you are already in the Family Mix but you don't see any tracks, try the following steps:
1. Leave the Family Mix
2. Log out off the app
3. Close the app
4. Open app and log back in
5. Search for the Family Mix playlist and join
If this post sums it up you can accept it as the solution, it might help someone else experiencing the same issue resolve it faster.