Insomnia is rarely fatal but if you do stay up too long and go for weeks without sleep then eventually you could die and go to sleep permanently.
The insomnia if left untreated can lead to health issues such as heart disease, heart attacks etc.
When you're sleep deprived you also can get into car accidents or walk out into traffic etc or have accidents at home that could be deadly.
Because you're not thinking clearly when you're deprived of sleep.
If you suffer from insomnia and cannot get to sleep after a few nights you should speak with your doctor and get some sleeping pills.
Sometimes over the counter sleep medicines can work but sometimes people need stronger sleep medicines that a doctor can prescribe.