What happens if you get caught in Singapore for five years?

It depends on the attitude of pleading guilty.

My classmate said over there that the immigration order is the most severe punishment (overdue for more than one year)

You will be sentenced to six months in prison, plus at least three lashes. Those who harbor them will also be convicted.

This is a serious crime, but you can still contact with the outside world. So if we lose contact, either the arrested person is embarrassed to call back, or something else has happened.