There are a lot of reasons your pet may suddenly become picky or finicky at mealtime. Sometimes it's as simple as they aren't hungry at their regular mealtime, or may be tired of the same food they usually eat. Other times, it could be pointing to an underlying health concern. First, don't panic. See if your pet will eat again at a different time of the day. Our pets really can be just like us - sometimes dinner time is earlier or later than usual.
While it isn't necessarily a problem that your pet doesn't eat at their mealtime, their weight is the thing to keep your eye on. If it's consistent, great - they're self-regulating and no need for concern. However, if you notice other physical signs such as lethargy, anxiety or any other sign of distress, it's a good idea to call your veterinarian and take your pet in for an exam.
Your pet may also be trying to tell you that they're tired of the food you're feeding them, or they may associate it with the reason they had an upset tummy, for example. Try a different food flavor. Consider keeping some food toppers on hand to sprinkle over their regular food. Though a topper won't replace their current food with a new flavor, it may be all the encouragement they need to clean their dish. I know that works for mine!