This red exclamation point means that the ArcMap project (.mxd) cannot locate your file. The project file (.mxd) is only a pointer to where the actual data is stored. You may have moved the individual files in manner that ArcMap cannot track (like in Windows Explorer). Double-click on the red exclamation point, and navigate to where your file is actually located. This redirects ArcMap so it ‘knows’ where your file is.
In the future, only move GIS files using ArcCatalog. Even using this, however, can sometimes confused ArcMap, so if you want your project to know where your files are, don't move them.