When you open a map, ArcMap looks for the data referenced
by each layer in the map. If it can't find the data source for a particular
layer, that layer won't be drawn. It
will display in the table of contents with a red exclamation mark next
to its name and the check box next to the layer will be unavailable.
If you know the new location of the data, you can repair the layer by doing the following:
Right-click on the layer name, point to Data, and click Set Data Source.
Navigate to the directory containing the correct data source file, click the file, then click Add.
ArcMap will attempt to find all data (if it is located in the same folder).
For more information, see ArcMap Help.