How do i create a level lock system?

Well i was working on my game and i have different scenes for different levels.
the goal of each level is to reach the finish line.

what i’ve done is i’ve created an empty gameobject and added a box collider to it.
so when ever the players hits the object my level completes.

what i wanted to do was to lock all levels except one and when someone completes the level the next level gets unlocked, So can any one help me with this ?

what i want is whenever players hit’s finish line next level gets unlock.
if possible can anyone post an example with just 2 level.

Just keep track of how many levels he has unlocked by using a PlayerPrefs int.