If you are unable to resolve the situation which causes you pressure or change your attitude towards it, the state of stress is prolonged and the negative effects of stress slowly manifest.
The increased mental focus due to stress becomes single-mindedness instead. The first thing to go is the feeling of spiritual connectedness with yourself and the world around you. Next, the range of emotions go down and all that's left are indifference, anxiety, anger and/or depression.
At this point, the stressed person is just a shell of his or her former self - a long rest is highly advised.
If that person still carries on with the stressed state, mental functions go down and the physical body starts to show discomfort. Discomfort slowly grows into pain, as the body has run out of other options to tell you "Hey, stop it already!".
If that's still not enough, things start to break. The sense of identity is fractured and the body becomes sick because it cannot take the imbalance anymore. In many cases the subconscious correctly judges the conscious mind to be the source of the problem and shuts it down. This is the final step it can take to stop the state of stress.
(If by any chance a person has awesome willpower along with the sheer stupidity to maintain the state of stress even beyond the state of grave illness, the body goes into system shock and dies.)