If you cannot get a thin, PTFE-based floss through the contact, use an end-cutting saw to carefully break through the very thin cement that is filling the contact. Avoid using diamond strips, which may end up opening the contact. In the future, be careful not to etch or splash self-etching adhesive onto adjacent tooth surfaces or resin-based restorations. If this is unavoidable, consider using commercially available PTFE-based tape to protect the adjacent surface during the tooth preparation phase of cementation.