First, you have to make sure that the nozzle isn’t clogged. Remove the nozzle from the can and run it under hot water. Then, with a clean rag, wipe away the softened paint. If the warm water doesn’t remove the paint, you can soak the nozzle in paint thinner overnight. Just make sure you wear protective gloves before you handle the thinner. In case the entire valve system is clogged, fill the spray can cap halfway with acetone (nail polish remover) and store the can upside down overnight. If this method doesn’t work, you can use compressed air to force the clogged paint out of the valve. In the future, to make sure that the spray can doesn’t get clogged, turn the can upside down and squirt it to clear any paint from the dip tube and nozzle when you're finished painting.