Scenario :

Why are the parts (wireframe & body) in Catia v5 appearing in black (or lock screen)?

Solutions:

There are different possibilities for this.
1. You might have applied material or default materiel property might be tweaked.

2. In View -> Lighting check whether single light source is checked.

3. Check Graphic Properties. (Most likely Graphic Property might be the reason. Check whether the colour is in Automatic)

4.When we click the lines on the branch it will get deep gray( or can tell Black). That's means the the the parts have got lucked.For unlock simple click on branch line.