Please look to make sure WMP is looking in the right location for your media.
Open WMP, and Right Click "Music".
Choose "Manage Music Library" and make sure it lists the folder where your Music is located.
If not, Click "ADD" and add the location you keep your Music.
If it does have the proper directories listed it may be corrupted for some reason. There are 2 ways to force WMP to rebuild its library.
-Right click the empty space at the top of WMP.
-Right Click and choose: Tools--> Advanced --> Restore Media Library
-Close WMP. Re-Open it and it should rebuild your library.
If this fails, Method 2)
-Make sure "Show Hidden" Files is enabled the Control Panel (In folder options)
-Go to the following location:
C:/ Users / your user name
/ AppData / Local / Microsoft / Media Player
-Inside the Media Player folder delete all the files, as shown below.
Re-Open WMP and it will now be forced to rebuild the entire library.