They often can't be bothered to do normal things like getting dressed or making something to eat. Sometimes mothers get depressed after a baby is born. This is called 'post-natal depression' or, if severe, 'puerperal psychosis'. This can make doing ordinary things very hard, for example, looking after the baby or other children. No-one really knows all the reasons why people become mentally ill.
Some people have a 'chemical imbalance' which affects how their brain works.
This makes them have strange thoughts or feelings, or behave oddly. They may need to take medication to help their brain work better. For other people, something might happen in their life which is very stressful, such as the death of someone very close, and this may trigger a mental illness. Mental illness doesn't normally start out of the blue. It usually develops slowly.