You have probably updated your angular and/or angular material versions; with the angular material 9 upgrade they changed the imports from @angular/material notation to @angular/material/button like notation.
Instead of importing from @angular/material, you should import deeply from the specific component. E.g. @angular/material/button. ng update will do this automatically for you.
You can follow the upgrade guide from this link:
https://update.angular.io/?v=8.0-9.0