Cant find file after revising tag

Windows Media Player in Windows 7 plays a file OK. After any change to the tag with MP3Tag, WMP can no longer find the file. If the file is copied and pasted in the same location, the copy is found and can play. Any idea why?

Do you really change the tags or do you change the filename?
You have to define a set of watched folders for WMP. WMP really watches these and notices any changes to the folders.
So, if you modify your file while WMP is open, WMP will immediately notice the change and update the library. This is probably why you saw the changes when you copied and pasted the file.
If you modify the file while WMP is closed you have to tell WMP to reread the folders.
(although, I think, this was the way up to WMP 10) In WMP11 you have to wait until the little arrow at the bottom does not say "Updating library" any more.

Any upates to TAGs should not lead to any hickups in WMP, as WMP stores the filename.
As long as the filename stays the same and the file has been registered by WMP it can be found and played.
As soon as you modify the filename you have to wait until WMP's update cycle comes around and rereads the file.
Annoyingly, WMP does not display all the updates to tags every time. Especially if you have modified tags in MWP it clings to that data stored in its own database and ignores widely any changes to the tags inside the files.

Oh, yes: editing files while WMP is open/running may lead to access violations because WMP has also grabbed a file in a folder....