Maybe she's giving you the silent treatment, or maybe she's told you that nothing's wrong in a tone of voice that's hardly convincing. Whatever the case may be, you know there's something bothering her that she's not telling you. Gently persuade her to open up to you by understanding and addressing the reasons she might be staying quiet in the first place.
Reflect on why she might be reluctant to talk about it. Women often strongly desire a form of emotional closeness where their partners can understand and anticipate their needs without having things explained to them. Try to figure out what's going on in her head, even if you can only partially guess.
Explain what your best guess is as to why she is upset and she may be more willing to open up to you all the way. If you're not sure, at least get into the general ballpark to show that you're trying. Guess with wide categories, such as, “Did something happen this morning?” or “Did I say the wrong thing?” She may be more likely to start opening up if she can begin with a “yes” or “no” reply.
Ask her questions about how she's feeling. Try to ask questions that get close to the issue, if you think you know what it is, such as "This was a stressful day, wasn't it?" or "You really wanted everything to go perfectly at this party, didn't you?" Gently encourage her to talk about what's on her mind and what she's feeling and be a good listener when she does.
Look for small ways that you can show understanding of her needs without being prompted. Do helpful, practical things for her that demonstrate this understanding, like assisting with chores. Even if you can't guess the specific issue, proving that you're sensitive to other issues may make her more likely to open up about what is wrong.
Give her your undivided attention for a while. Be attentive in a noninvasive way that shows that you are available and interested in what she's feeling. Respond to what she has to say, and avoid changing topics too early, giving short answers to her questions or appearing distracted. Be emotionally available to help her feel comfortable opening up.
Open up about some of your own feelings, if it seems appropriate and welcome. If she feels like opening up would make her too vulnerable, some vulnerability from you will equalize the situation, even if you're opening up about something that isn't related to the matter at hand.
Ask her directly, if all else fails and if you believe you have a right to know what's on her mind. Tell her that you'd like to understand why it is that she isn't telling you what's going on and that you've done everything you can to make her feel comfortable enough to open up. Explain that you need her to take the next step and come forward.