Cotty’s questions are essential to provide an answer particularly if you can share the model.
If you have truly lost the faces and they are not just hidden or on a hidden layer you could however try the plug in Eneroth Face Creator. Unlike some of the other face creation plugins it will also work on components without exploding them.
https://extensions.sketchup.com/en/content/eneroth-face-creator
If that doesn’t work,as a lost resort the Sandbox tool could even be tried.