This is something that can happen when the scene file is saved (or moved) to a location where it no longer points to the drawings/levels referenced in the Project.
The worst case scenario is that the drawings/levels were not saved and do not exist to reference. We will assume (and hope) that is not the case with your Project.
The most likely place for the files to be would be in the Toonz stuff folder located (for Windows users) at the follow locations:
C:\Opentoonz 1.1 stuff\sandbox\scenes
C:\Opentonnz 1.1 stuff\Projects\ your project file name here
C:\Opentoonz 1.1 stuff\sandbox\drawings
Look in the various folders at this general location for your files.
If you Right Click in the xsheet on the name/cell that is missing you should see a Level Settings option.
You can point that reference to the proper location... if you know the location of the file.
Another approach to resolving the problem of broken references would be to move the Scene (.tnz file) to a location where it automatically will resolve the broken links.
This is a good approach to take if you recently manually moved a project or scene file to another folder.
Think of this like working with hypertext markup (html) where we might have a webpage that looks great but then we move the images folder (and all of its contents) to a different location. At the point we move the files... the link is broken and our webpage no longer can find the files to display them properly.
A good way to trouble shoot missing files would be to examine where the missing file reference is currently expecting to find the file.
Hopefully that will give a clue as to where the files use to reside so they can be relocated.
Good luck!