PROBLEM:
Levels are not visible in sections or elevations, but you know they exist.
CAUSE:
The Project Base Point is a long way from the model.
Effectively the Levels don't reach the model.
SOLUTION:
Go to a plan and zoom out so it includes the model and the Project Base Point (or the area where you think the levels were created).
Create a Scope Box - View tab, Scope Box.
Make is big enough to include everything including the Project Base Point.
Give the Scope Box a name.
Create a section through the scope box.
Go to that section and drag the Scope Box's top and bottom boundaries so the box contains all model elements (and where you believe the levels are).
Draw a new level in elevation by either creating a new level or just copying an existing one up or down.
Right click on the newly created level and choose to Select all instances > In Project.
Go to Properties and set them to apply to the scope box you just created.
Your levels should now all be visible. You can delete the scope box, new level and section if you no longer need them.
PREVENTION:
Don't create project files where the Project Base Point is a long way from the model.