Hi - which OS are we talking about because WMP12 is part of the system these days. I don't think your TomTom device will be involved... it's more likely a Windows update issue if you are certain you have not altered or moved anything. Rebuilding the library will only serve to restore broken links, but let's check...
Open media player - find one of the songs that won't play... check that it still won't play. Then right-click it and select 'Open file location' - if the track opens in 'explorer' it is not a library link problem. Then with the track opened in explorer, select it there - right click and select 'Play'. If that also fails it's definitely a system problem.
Update when you have tried these tests. - Thanks, Ric.