Exercises are unlocked differently based on the type of track:
Requiring Learning Exercises to be completed to unlock more exercises is called Learning Mode (which was introduced with Exercism 3.0). Learning Mode is the default for any user that joins a track with Learning Exercises, but also for any users from the old site. Practice Mode is the opposite, where all exercises are unlocked from the start.
Note that in either Practice or Learning Mode, one can ask for mentoring on a particular exercise.
If you are a returning user, who used a previous version of Exercism, you may find that exercises that were previously available have now become locked. While we recommend staying in Learning Mode and solving Learning Exercises to unlock other exercises (each should take around 5 minutes to solve if you are fluent in a language), you could instead switch from Learning Mode to Practice Mode.
To switch modes, go to the Track Page of the track you've joined and click on the "..." icon on the top right hand side. Then, depending on your current mode, select either "Disable Learning Mode" or "Enable Learning Mode" and confirm that you want to switch modes.
Note that it currently takes a while for the current mode to be reported in your Tracks page.