Cecilia,
Unless you can find a new carbon button and any of the little carbon disks you need, that foot controller is toast. It's best to just replace it. BUT --- Do not throw it away. Put it in a zip lock bag and keep it for parts. For as sure as can be if you toss it, in due time you'll need those little carbon disks for another one. That is exactly what happened to me. One controller died. Replaced it with a new one, tossed it. Not 6 months later I bought a nice cute little SEWMOR that had a damaged controller. I managed to find some carbon disks thanks to another forum member, and now it's up and running. But without them, I would have had to replace another one.
Joe