Well, this depends. If you were saying no because you honestly believe that you cannot accommodate your manager's (or whatever) request into your current schedu

Well, this depends. If you were saying no because you honestly believe that you cannot accommodate your manager's (or whatever) request into your current schedule/plan/loading, then you probably want to take the time to get in writing a new plan that exactly articulates what your manager believes your old plan and priorities were, and what your new plan and priorities are. You are now trying to solve a planning and communication problem. If you were just saying no because you didn't -want- to do it. Well, then your manager is calling your bluff, and ya, you need to find another way to wiggle out of it. Tough luck.