First, admit to yourself that there’s a problem. Think about the parameters of the problem and how it’s impacting your work productivity. Also, consider what your role—and the other person’s role—in causing the problem may be, since all relationships are a two-way street.
Next, take the person in question aside. Explain that there’s an issue between you that needs addressing—and you’d like the opportunity to do so. Set up a meeting to discuss the issue. Make sure you meet in a private, neutral space, say an empty office or a conference room.
Spend time considering what you’ll say at the meeting. Don’t plan to assign blame, since this will defeat your purpose of a mutually beneficial discussion. Think about any positive aspects of your relationship you can build on in the future, and ways each person might change their behavior to ameliorate the issue.
At the meeting, keep your emotions in check. Again, don’t argue about what went wrong or who said what. Identify actions you can both take to improve the relationship. Focus on what should be your shared objective: to work in harmony to make your department, and your organization, stronger and more successful.
Remember three things: First, you’re on the same side. Second, neither of you are mind readers, which makes ongoing communication vital. Third, repairing relationships takes time.
Rebuilding trust doesn’t happen overnight. However, if you’re both committed to change, things will get better. Have patience. Expect setbacks, and roll with them. Make an effort to improve your attitude toward the other person and the tenor of your interactions.
Last, but not least, keep in mind that this is a professional relationship; it’s not necessary to be fast friends—only to be respectful of each other and work well together.