Ischemia is a shortage of oxygenated blood to the heart and is not a disease but an effect or coronary disease.
Healthy coronary arteries can carry much more blood to the heart than the heart ever needs. Coronary disease can develop over many years and when the blockages in the coronary arteries become severe enough, the blood supply is not sufficient anymore (especially during exercise, when there is more blood required or during stress or cold when the arteries narrow).
That is ischemia, and it often comes with symptoms like chest discomfort.
In this stage, there is not much needed for the heart getting into big trouble from lack of oxygen and a fatal heart attack can happen.