What happens if a computer has a virus that cant be deleted?

Then the computer is broken. But what you can do is wipe everything on it and that kills the virus. If it's a blue screen of death then you need a computer genius to type some code and delete the blue screen of death. However if it is a Trojan virus there is no way of get tut rid of it

It depends on what kind of virus the computer is infected.

Adware, Backdoor, Dialler, Hijackers, Spyware ...