Right click on the layer in table of contents.
Select data> repair data source.
Then save the layer file again.
There could be issues with either your MXD location, or you relative/actual path setting.
I use the relative path method but ALWAYS keep my MXD in a folder directly under the data.
always keep the mxd file in the folder together
Not exactly (but I have hundreds of long life documents (mxd)). I use the relative depicted in the link except that my mxd folder is "in-line" with my data folder.
"Maps" being where my mxd files are.
The bottome portion of the image is not what I do either.
I use a region/project folder structure , and then split out my data types inside the project.
With basemaps being at the root data level.
That is just what works for me. I can come back to documents years later, if the data has been moved I can still repair and it will repair "all" data in the document easily.
The point of the relative and absolute options are that each user can use it differently.
As-per their needs.