If you have heart failure, it does not mean that your heart has stopped or is about to stop working.
It means that your heart is not able to pump blood the way it should. The condition is called "congestive" heart failure (CHF) when this inability leads to heart failure symptoms, such as:
(For more info, click Heart Failure. This article includes more detailed information about congestive heart failure symptoms and how the condition progresses, as well as statistics on how many people it affects.)