And for whatever reason, you may miss feedings. It is important to catch it as early as possible, and it is important to know how to treat it - just in case. The key to relieving engorgement is removing the milk. Sometimes it isn't easy to do, though. When there is so much milk and extra fluid in the breasts, this can put pressure on the milk ducts such that they close down - like standing on a garden hose.
Compresses are an effective method of helping that milk to flow.
As a general rule, heat gets the milk flowing, and cold reduces swelling. Apply hot compresses to your breasts before nursing, and cold compresses after nursing. Disposable diapers make wonderful compresses, by the way. Sometimes, even with hot compresses, there is so much tissue swelling that the milk still won't flow. If this happens, try using a cold compress for 10 minutes or so and then wait 20 minutes or so and try the heat again.