As an employee in public relations, how do you build credibility to a company that once lost it?

Have your work speaks for you first and foremost by all means. Convince your company to give them a better rates as a 2nd chance for reconciliation.  

Stand on the principles of Integrity and Honesty; ultimately the Truth will be triumphant and prevails.

You need to act promptly, work hard to prove and demonstrate publicly that your organisation have Changed Now and apologies using all channels of public and social media to all those stakeholders who suffered.

Personal letters to each customer would bring back the lost confidence and credibility.