@Christina Holloway - What plan type does your colleague have?
How exactly did they add/invite you as an editor? E.g., are they using Visitor/"Anyone with the link" access or did they open the board's Share settings and enter your email address at the very top of that screen and add you as a team member or a Guest?
Did they use the same email address that you are signing in with now?
Are they a Member of their team or are they an Admin? Perhaps they were trying to invite you to the board as a guest but added you as a full team member (using up a paid seat, or increasing the number of pairs seats by one) and an admin has since removed you.
What happens if they just send you a link to the board and you try to open it?