Trust Will Be Broken
You may not want to hear about this, but it is an absolute fact that sometimes you will trust people, and they will let you down. It has to happen. If it didn't happen, then trust would not really be an issue. As I heard someone say once, if you caught a fish every time, they would call it catching, not fishing, now wouldn't they? Having trust violated is hard. Because for trust to really be violated, it has to have been sustained for a while. In other words, you trust someone to be competent or to protect your interests, and for a while they are doing just that--it feels great. And then comes the day when they let you down. Not so great.
As leaders, you need to prepare yourselves for this eventuality, because by definition, it's going to catch you off guard. This person, or department, or unit, or organization, will obviously have been doing great (hence the trust), and then you come into the office one day to discover that they completely let you down. Get ready for the emotional reaction you will have. Get ready to be pissed off. Let that all work through you, and resist the temptation to email them at that particular moment. You're not going to figure a way out of this at the precise moment when you're most angry.
And depending on the relationship you have with the other person or group, prepare for the "I'm sure this was just a one-time thing and let's just overlook it" reaction. When you're close, it's easy to write off the trust violation and focus on the history of goodwill. Let that work through you as well, because it can be equally misguiding. Ignoring a serious violation of trust never works out in the long run.
What you need is a new relationship. You're not restarting from scratch, but a lot of how you work together is now being rewritten. It may not feel fair, or the way you want it, but the reality is in order to rebuild trust, you lower the bar a bit. You ratchet down the kinds of risks you are willing to take with the other party. Maybe they need to report things to you more often, or get approvals on decisions they used to make themselves. It's going to be extra work, on both parts. It will slow things down. But if it's a relationship worth keeping (and most are), then you'll do this work to build it back up.
Trust is a beautiful thing because it allows you to let go of your worry about certain outcomes. When it's really working, it allows you to take some things (and some people and groups) for granted--but in a good way. This is how trust enables speed. So to get back to this state after trust has been violated, you have to go back to the stage where people are NOT taken for granted. Where the building of trust is intentional and visible. This is the work of building trust--showing each other in small and tangible ways that you can trust each other. It will feel odd if this relationship was very high in trust before. But that's the work of building trust, and there isn't a shortcut. You bring the relationship back into focus for a period of time. You get clarity on both sides about what trust is and why it matters. And soon you'll be back to where you were.
Stay centered. Clearly express the impact of the trust violation to the other party, and then equally clearly express your intention to rebuild the trust. Then do the work of rebuilding trust.